Increase the Zippy block size from 32 kB to 64 kB, winning ~3% density
while being effectively performance neutral.

The longer story about density is that we win 3-6% density on the benchmarks 
where this has any effect at all; many of the benchmarks (cp, c, lsp, man)
are smaller than 32 kB and thus will have no effect. Binary data also seems
to win little or nothing; of course, the already-compressed data wins nothing.
The protobuf benchmark wins as much as ~18% depending on architecture,
but I wouldn't be too sure that this is representative of protobuf data in
general.

As of performance, we lose a tiny amount since we get more tags (e.g., a long
literal might be broken up into literal-copy-literal), but we win it back with
less clearing of the hash table, and more opportunities to skip incompressible
data (e.g. in the jpg benchmark). Decompression seems to get ever so slightly
slower, again due to more tags. The total net change is about as close to zero
as we can get, so the end effect seems to be simply more density and no
real performance change.

The comment about not changing kBlockSize, scary as it is, is not really
relevant, since we're never going to have a block-level decompressor without
explicitly marked blocks. Replace it with something more appropriate.

This affects the framing format, but it's okay to change it since it basically
has no users yet.


Density (note that cp, c, lsp and man are all smaller than 32 kB):

   Benchmark         Description   Base (%)  New (%)  Improvement
   --------------------------------------------------------------
   ZFlat/0           html            22.57    22.31     +5.6%
   ZFlat/1           urls            50.89    47.77     +6.5%
   ZFlat/2           jpg             99.88    99.87     +0.0%
   ZFlat/3           pdf             82.13    82.07     +0.1%
   ZFlat/4           html4           23.55    22.51     +4.6%
   ZFlat/5           cp              48.12    48.12     +0.0%
   ZFlat/6           c               42.40    42.40     +0.0%
   ZFlat/7           lsp             48.37    48.37     +0.0%
   ZFlat/8           xls             41.34    41.23     +0.3%
   ZFlat/9           txt1            59.81    57.87     +3.4%
   ZFlat/10          txt2            64.07    61.93     +3.5%
   ZFlat/11          txt3            57.11    54.92     +4.0%
   ZFlat/12          txt4            68.35    66.22     +3.2%
   ZFlat/13          bin             18.21    18.11     +0.6%
   ZFlat/14          sum             51.88    48.96     +6.0%
   ZFlat/15          man             59.36    59.36     +0.0%
   ZFlat/16          pb              23.15    19.64    +17.9%
   ZFlat/17          gaviota         38.27    37.72     +1.5%
   Geometric mean                    45.51    44.15     +3.1%


Microbenchmarks (64-bit, opt):

Westmere 2.8 GHz:

   Benchmark                          Base (ns)  New (ns)                                Improvement
   -------------------------------------------------------------------------------------------------
   BM_UFlat/0                             75342     75027  1.3GB/s  html                    +0.4%
   BM_UFlat/1                            723767    744269  899.6MB/s  urls                  -2.8%
   BM_UFlat/2                             10072     10072  11.7GB/s  jpg                    +0.0%
   BM_UFlat/3                             30747     30388  2.9GB/s  pdf                     +1.2%
   BM_UFlat/4                            307353    306063  1.2GB/s  html4                   +0.4%
   BM_UFlat/5                             28593     28743  816.3MB/s  cp                    -0.5%
   BM_UFlat/6                             12958     12998  818.1MB/s  c                     -0.3%
   BM_UFlat/7                              3700      3792  935.8MB/s  lsp                   -2.4%
   BM_UFlat/8                            999685    999905  982.1MB/s  xls                   -0.0%
   BM_UFlat/9                            232954    230079  630.4MB/s  txt1                  +1.2%
   BM_UFlat/10                           200785    201468  592.6MB/s  txt2                  -0.3%
   BM_UFlat/11                           617267    610968  666.1MB/s  txt3                  +1.0%
   BM_UFlat/12                           821595    822475  558.7MB/s  txt4                  -0.1%
   BM_UFlat/13                           377097    377632  1.3GB/s  bin                     -0.1%
   BM_UFlat/14                            45476     45260  805.8MB/s  sum                   +0.5%
   BM_UFlat/15                             4985      5003  805.7MB/s  man                   -0.4%
   BM_UFlat/16                            80813     77494  1.4GB/s  pb                      +4.3%
   BM_UFlat/17                           251792    241553  727.7MB/s  gaviota               +4.2%
   BM_UValidate/0                         40343     40354  2.4GB/s  html                    -0.0%
   BM_UValidate/1                        426890    451574  1.4GB/s  urls                    -5.5%
   BM_UValidate/2                           187       179  661.9GB/s  jpg                   +4.5%
   BM_UValidate/3                         13783     13827  6.4GB/s  pdf                     -0.3%
   BM_UValidate/4                        162393    163335  2.3GB/s  html4                   -0.6%
   BM_UDataBuffer/0                       93756     93302  1046.7MB/s  html                 +0.5%
   BM_UDataBuffer/1                      886714    916292  730.7MB/s  urls                  -3.2%
   BM_UDataBuffer/2                       15861     16401  7.2GB/s  jpg                     -3.3%
   BM_UDataBuffer/3                       38934     39224  2.2GB/s  pdf                     -0.7%
   BM_UDataBuffer/4                      381008    379428  1029.5MB/s  html4                +0.4%
   BM_UCord/0                             92528     91098  1072.0MB/s  html                 +1.6%
   BM_UCord/1                            858421    885287  756.3MB/s  urls                  -3.0%
   BM_UCord/2                             13140     13464  8.8GB/s  jpg                     -2.4%
   BM_UCord/3                             39012     37773  2.3GB/s  pdf                     +3.3%
   BM_UCord/4                            376869    371267  1052.1MB/s  html4                +1.5%
   BM_UCordString/0                       75810     75303  1.3GB/s  html                    +0.7%
   BM_UCordString/1                      735290    753841  888.2MB/s  urls                  -2.5%
   BM_UCordString/2                       11945     13113  9.0GB/s  jpg                     -8.9%
   BM_UCordString/3                       33901     32562  2.7GB/s  pdf                     +4.1%
   BM_UCordString/4                      310985    309390  1.2GB/s  html4                   +0.5%
   BM_UCordValidate/0                     40952     40450  2.4GB/s  html                    +1.2%
   BM_UCordValidate/1                    433842    456531  1.4GB/s  urls                    -5.0%
   BM_UCordValidate/2                      1179      1173  100.8GB/s  jpg                   +0.5%
   BM_UCordValidate/3                     14481     14392  6.1GB/s  pdf                     +0.6%
   BM_UCordValidate/4                    164364    164151  2.3GB/s  html4                   +0.1%
   BM_ZFlat/0                            160610    156601  623.6MB/s  html (22.31 %)        +2.6%
   BM_ZFlat/1                           1995238   1993582  335.9MB/s  urls (47.77 %)        +0.1%
   BM_ZFlat/2                             30133     24983  4.7GB/s  jpg (99.87 %)          +20.6%
   BM_ZFlat/3                             74453     73128  1.2GB/s  pdf (82.07 %)           +1.8%
   BM_ZFlat/4                            647674    633729  616.4MB/s  html4 (22.51 %)       +2.2%
   BM_ZFlat/5                             76259     76090  308.4MB/s  cp (48.12 %)          +0.2%
   BM_ZFlat/6                             31106     31084  342.1MB/s  c (42.40 %)           +0.1%
   BM_ZFlat/7                             10507     10443  339.8MB/s  lsp (48.37 %)         +0.6%
   BM_ZFlat/8                           1811047   1793325  547.6MB/s  xls (41.23 %)         +1.0%
   BM_ZFlat/9                            597903    581793  249.3MB/s  txt1 (57.87 %)        +2.8%
   BM_ZFlat/10                           525320    514522  232.0MB/s  txt2 (61.93 %)        +2.1%
   BM_ZFlat/11                          1596591   1551636  262.3MB/s  txt3 (54.92 %)        +2.9%
   BM_ZFlat/12                          2134523   2094033  219.5MB/s  txt4 (66.22 %)        +1.9%
   BM_ZFlat/13                           593024    587869  832.6MB/s  bin (18.11 %)         +0.9%
   BM_ZFlat/14                           114746    110666  329.5MB/s  sum (48.96 %)         +3.7%
   BM_ZFlat/15                            14376     14485  278.3MB/s  man (59.36 %)         -0.8%
   BM_ZFlat/16                           167908    150070  753.6MB/s  pb (19.64 %)         +11.9%
   BM_ZFlat/17                           460228    442253  397.5MB/s  gaviota (37.72 %)     +4.1%
   BM_ZCord/0                            164896    160241  609.4MB/s  html                  +2.9%
   BM_ZCord/1                           2070239   2043492  327.7MB/s  urls                  +1.3%
   BM_ZCord/2                             54402     47002  2.5GB/s  jpg                    +15.7%
   BM_ZCord/3                             85871     83832  1073.1MB/s  pdf                  +2.4%
   BM_ZCord/4                            664078    648825  602.0MB/s  html4                 +2.4%
   BM_ZDataBuffer/0                      174874    172549  566.0MB/s  html                  +1.3%
   BM_ZDataBuffer/1                     2134410   2139173  313.0MB/s  urls                  -0.2%
   BM_ZDataBuffer/2                       71911     69551  1.7GB/s  jpg                     +3.4%
   BM_ZDataBuffer/3                       98236     99727  902.1MB/s  pdf                   -1.5%
   BM_ZDataBuffer/4                      710776    699104  558.8MB/s  html4                 +1.7%
   Sum of all benchmarks               27358908  27200688                                   +0.6%


Sandy Bridge 2.6 GHz:

   Benchmark                          Base (ns)  New (ns)                                Improvement
   -------------------------------------------------------------------------------------------------
   BM_UFlat/0                             49356     49018  1.9GB/s  html                    +0.7%
   BM_UFlat/1                            516764    531955  1.2GB/s  urls                    -2.9%
   BM_UFlat/2                              6982      7304  16.2GB/s  jpg                    -4.4%
   BM_UFlat/3                             15285     15598  5.6GB/s  pdf                     -2.0%
   BM_UFlat/4                            206557    206669  1.8GB/s  html4                   -0.1%
   BM_UFlat/5                             13681     13567  1.7GB/s  cp                      +0.8%
   BM_UFlat/6                              6571      6592  1.6GB/s  c                       -0.3%
   BM_UFlat/7                              2008      1994  1.7GB/s  lsp                     +0.7%
   BM_UFlat/8                            775700    773286  1.2GB/s  xls                     +0.3%
   BM_UFlat/9                            165578    164480  881.8MB/s  txt1                  +0.7%
   BM_UFlat/10                           143707    144139  828.2MB/s  txt2                  -0.3%
   BM_UFlat/11                           443026    436281  932.8MB/s  txt3                  +1.5%
   BM_UFlat/12                           603129    595856  771.2MB/s  txt4                  +1.2%
   BM_UFlat/13                           271682    270450  1.8GB/s  bin                     +0.5%
   BM_UFlat/14                            26200     25666  1.4GB/s  sum                     +2.1%
   BM_UFlat/15                             2620      2608  1.5GB/s  man                     +0.5%
   BM_UFlat/16                            48908     47756  2.3GB/s  pb                      +2.4%
   BM_UFlat/17                           174638    170346  1031.9MB/s  gaviota              +2.5%
   BM_UValidate/0                         31922     31898  3.0GB/s  html                    +0.1%
   BM_UValidate/1                        341265    363554  1.8GB/s  urls                    -6.1%
   BM_UValidate/2                           160       151  782.8GB/s  jpg                   +6.0%
   BM_UValidate/3                         10402     10380  8.5GB/s  pdf                     +0.2%
   BM_UValidate/4                        129490    130587  2.9GB/s  html4                   -0.8%
   BM_UDataBuffer/0                       59383     58736  1.6GB/s  html                    +1.1%
   BM_UDataBuffer/1                      619222    637786  1049.8MB/s  urls                 -2.9%
   BM_UDataBuffer/2                       10775     11941  9.9GB/s  jpg                     -9.8%
   BM_UDataBuffer/3                       18002     17930  4.9GB/s  pdf                     +0.4%
   BM_UDataBuffer/4                      259182    259306  1.5GB/s  html4                   -0.0%
   BM_UCord/0                             59379     57814  1.6GB/s  html                    +2.7%
   BM_UCord/1                            598456    615162  1088.4MB/s  urls                 -2.7%
   BM_UCord/2                              8519      8628  13.7GB/s  jpg                    -1.3%
   BM_UCord/3                             18123     17537  5.0GB/s  pdf                     +3.3%
   BM_UCord/4                            252375    252331  1.5GB/s  html4                   +0.0%
   BM_UCordString/0                       49494     49790  1.9GB/s  html                    -0.6%
   BM_UCordString/1                      524659    541803  1.2GB/s  urls                    -3.2%
   BM_UCordString/2                        8206      8354  14.2GB/s  jpg                    -1.8%
   BM_UCordString/3                       17235     16537  5.3GB/s  pdf                     +4.2%
   BM_UCordString/4                      210188    211072  1.8GB/s  html4                   -0.4%
   BM_UCordValidate/0                     31956     31587  3.0GB/s  html                    +1.2%
   BM_UCordValidate/1                    340828    362141  1.8GB/s  urls                    -5.9%
   BM_UCordValidate/2                       783       744  158.9GB/s  jpg                   +5.2%
   BM_UCordValidate/3                     10543     10462  8.4GB/s  pdf                     +0.8%
   BM_UCordValidate/4                    130150    129789  2.9GB/s  html4                   +0.3%
   BM_ZFlat/0                            113873    111200  878.2MB/s  html (22.31 %)        +2.4%
   BM_ZFlat/1                           1473023   1489858  449.4MB/s  urls (47.77 %)        -1.1%
   BM_ZFlat/2                             23569     19486  6.1GB/s  jpg (99.87 %)          +21.0%
   BM_ZFlat/3                             49178     48046  1.8GB/s  pdf (82.07 %)           +2.4%
   BM_ZFlat/4                            475063    469394  832.2MB/s  html4 (22.51 %)       +1.2%
   BM_ZFlat/5                             46910     46816  501.2MB/s  cp (48.12 %)          +0.2%
   BM_ZFlat/6                             16883     16916  628.6MB/s  c (42.40 %)           -0.2%
   BM_ZFlat/7                              5381      5447  651.5MB/s  lsp (48.37 %)         -1.2%
   BM_ZFlat/8                           1466870   1473861  666.3MB/s  xls (41.23 %)         -0.5%
   BM_ZFlat/9                            468006    464101  312.5MB/s  txt1 (57.87 %)        +0.8%
   BM_ZFlat/10                           408157    408957  291.9MB/s  txt2 (61.93 %)        -0.2%
   BM_ZFlat/11                          1253348   1232910  330.1MB/s  txt3 (54.92 %)        +1.7%
   BM_ZFlat/12                          1702373   1702977  269.8MB/s  txt4 (66.22 %)        -0.0%
   BM_ZFlat/13                           439792    438557  1116.0MB/s  bin (18.11 %)        +0.3%
   BM_ZFlat/14                            80766     78851  462.5MB/s  sum (48.96 %)         +2.4%
   BM_ZFlat/15                             7420      7542  534.5MB/s  man (59.36 %)         -1.6%
   BM_ZFlat/16                           112043    100126  1.1GB/s  pb (19.64 %)           +11.9%
   BM_ZFlat/17                           368877    357703  491.4MB/s  gaviota (37.72 %)     +3.1%
   BM_ZCord/0                            116402    113564  859.9MB/s  html                  +2.5%
   BM_ZCord/1                           1507156   1519911  440.5MB/s  urls                  -0.8%
   BM_ZCord/2                             39860     33686  3.5GB/s  jpg                    +18.3%
   BM_ZCord/3                             56211     54694  1.6GB/s  pdf                     +2.8%
   BM_ZCord/4                            485594    479212  815.1MB/s  html4                 +1.3%
   BM_ZDataBuffer/0                      123185    121572  803.3MB/s  html                  +1.3%
   BM_ZDataBuffer/1                     1569111   1589380  421.3MB/s  urls                  -1.3%
   BM_ZDataBuffer/2                       53143     49556  2.4GB/s  jpg                     +7.2%
   BM_ZDataBuffer/3                       65725     66826  1.3GB/s  pdf                     -1.6%
   BM_ZDataBuffer/4                      517871    514750  758.9MB/s  html4                 +0.6%
   Sum of all benchmarks               20258879  20315484                                   -0.3%


AMD Instanbul 2.4 GHz:

   Benchmark                          Base (ns)  New (ns)                                Improvement
   -------------------------------------------------------------------------------------------------
   BM_UFlat/0                             97120     96585  1011.1MB/s  html                 +0.6%
   BM_UFlat/1                            917473    948016  706.3MB/s  urls                  -3.2%
   BM_UFlat/2                             21496     23938  4.9GB/s  jpg                    -10.2%
   BM_UFlat/3                             44751     45639  1.9GB/s  pdf                     -1.9%
   BM_UFlat/4                            391950    391413  998.0MB/s  html4                 +0.1%
   BM_UFlat/5                             37366     37201  630.7MB/s  cp                    +0.4%
   BM_UFlat/6                             18350     18318  580.5MB/s  c                     +0.2%
   BM_UFlat/7                              5672      5661  626.9MB/s  lsp                   +0.2%
   BM_UFlat/8                           1533390   1529441  642.1MB/s  xls                   +0.3%
   BM_UFlat/9                            335477    336553  431.0MB/s  txt1                  -0.3%
   BM_UFlat/10                           285140    292080  408.7MB/s  txt2                  -2.4%
   BM_UFlat/11                           888507    894758  454.9MB/s  txt3                  -0.7%
   BM_UFlat/12                          1187643   1210928  379.5MB/s  txt4                  -1.9%
   BM_UFlat/13                           493717    507447  964.5MB/s  bin                   -2.7%
   BM_UFlat/14                            61740     60870  599.1MB/s  sum                   +1.4%
   BM_UFlat/15                             7211      7187  560.9MB/s  man                   +0.3%
   BM_UFlat/16                            97435     93100  1.2GB/s  pb                      +4.7%
   BM_UFlat/17                           362662    356395  493.2MB/s  gaviota               +1.8%
   BM_UValidate/0                         47475     47118  2.0GB/s  html                    +0.8%
   BM_UValidate/1                        501304    529741  1.2GB/s  urls                    -5.4%
   BM_UValidate/2                           276       243  486.2GB/s  jpg                  +13.6%
   BM_UValidate/3                         16361     16261  5.4GB/s  pdf                     +0.6%
   BM_UValidate/4                        190741    190353  2.0GB/s  html4                   +0.2%
   BM_UDataBuffer/0                      111080    109771  889.6MB/s  html                  +1.2%
   BM_UDataBuffer/1                     1051035   1085999  616.5MB/s  urls                  -3.2%
   BM_UDataBuffer/2                       25801     25463  4.6GB/s  jpg                     +1.3%
   BM_UDataBuffer/3                       50493     49946  1.8GB/s  pdf                     +1.1%
   BM_UDataBuffer/4                      447258    444138  879.5MB/s  html4                 +0.7%
   BM_UCord/0                            109350    107909  905.0MB/s  html                  +1.3%
   BM_UCord/1                           1023396   1054964  634.7MB/s  urls                  -3.0%
   BM_UCord/2                             25292     24371  4.9GB/s  jpg                     +3.8%
   BM_UCord/3                             48955     49736  1.8GB/s  pdf                     -1.6%
   BM_UCord/4                            440452    437331  893.2MB/s  html4                 +0.7%
   BM_UCordString/0                       98511     98031  996.2MB/s  html                  +0.5%
   BM_UCordString/1                      933230    963495  694.9MB/s  urls                  -3.1%
   BM_UCordString/2                       23311     24076  4.9GB/s  jpg                     -3.2%
   BM_UCordString/3                       45568     46196  1.9GB/s  pdf                     -1.4%
   BM_UCordString/4                      397791    396934  984.1MB/s  html4                 +0.2%
   BM_UCordValidate/0                     47537     46921  2.0GB/s  html                    +1.3%
   BM_UCordValidate/1                    505071    532716  1.2GB/s  urls                    -5.2%
   BM_UCordValidate/2                      1663      1621  72.9GB/s  jpg                    +2.6%
   BM_UCordValidate/3                     16890     16926  5.2GB/s  pdf                     -0.2%
   BM_UCordValidate/4                    192365    191984  2.0GB/s  html4                   +0.2%
   BM_ZFlat/0                            184708    179103  545.3MB/s  html (22.31 %)        +3.1%
   BM_ZFlat/1                           2293864   2302950  290.7MB/s  urls (47.77 %)        -0.4%
   BM_ZFlat/2                             52852     47618  2.5GB/s  jpg (99.87 %)          +11.0%
   BM_ZFlat/3                            100766     96179  935.3MB/s  pdf (82.07 %)         +4.8%
   BM_ZFlat/4                            741220    727977  536.6MB/s  html4 (22.51 %)       +1.8%
   BM_ZFlat/5                             85402     85418  274.7MB/s  cp (48.12 %)          -0.0%
   BM_ZFlat/6                             36558     36494  291.4MB/s  c (42.40 %)           +0.2%
   BM_ZFlat/7                             12706     12507  283.7MB/s  lsp (48.37 %)         +1.6%
   BM_ZFlat/8                           2336823   2335688  420.5MB/s  xls (41.23 %)         +0.0%
   BM_ZFlat/9                            701804    681153  212.9MB/s  txt1 (57.87 %)        +3.0%
   BM_ZFlat/10                           606700    597194  199.9MB/s  txt2 (61.93 %)        +1.6%
   BM_ZFlat/11                          1852283   1803238  225.7MB/s  txt3 (54.92 %)        +2.7%
   BM_ZFlat/12                          2475527   2443354  188.1MB/s  txt4 (66.22 %)        +1.3%
   BM_ZFlat/13                           694497    696654  702.6MB/s  bin (18.11 %)         -0.3%
   BM_ZFlat/14                           136929    129855  280.8MB/s  sum (48.96 %)         +5.4%
   BM_ZFlat/15                            17172     17124  235.4MB/s  man (59.36 %)         +0.3%
   BM_ZFlat/16                           190364    171763  658.4MB/s  pb (19.64 %)         +10.8%
   BM_ZFlat/17                           567285    555190  316.6MB/s  gaviota (37.72 %)     +2.2%
   BM_ZCord/0                            193490    187031  522.1MB/s  html                  +3.5%
   BM_ZCord/1                           2427537   2415315  277.2MB/s  urls                  +0.5%
   BM_ZCord/2                             85378     81412  1.5GB/s  jpg                     +4.9%
   BM_ZCord/3                            121898    119419  753.3MB/s  pdf                   +2.1%
   BM_ZCord/4                            779564    762961  512.0MB/s  html4                 +2.2%
   BM_ZDataBuffer/0                      213820    207272  471.1MB/s  html                  +3.2%
   BM_ZDataBuffer/1                     2589010   2586495  258.9MB/s  urls                  +0.1%
   BM_ZDataBuffer/2                      121871    118885  1018.4MB/s  jpg                  +2.5%
   BM_ZDataBuffer/3                      145382    145986  616.2MB/s  pdf                   -0.4%
   BM_ZDataBuffer/4                      868117    852754  458.1MB/s  html4                 +1.8%
   Sum of all benchmarks               33771833  33744763                                   +0.1%