Make the two IncrementalCopy* functions take in an ssize_t instead of a len,
in order to avoid having to do 32-to-64-bit signed conversions on a hot path
during decompression. (Also fixes some MSVC warnings, mentioned in public
issue 75, but more of those remain.) They cannot be size_t because we expect
them to go negative and test for that.

This saves a few movzwl instructions, yielding ~2% speedup in decompression.


Sandy Bridge:

Benchmark                          Base (ns)  New (ns)                                Improvement
-------------------------------------------------------------------------------------------------
BM_UFlat/0                             48009     41283  2.3GB/s  html                   +16.3%
BM_UFlat/1                            531274    513419  1.3GB/s  urls                    +3.5%
BM_UFlat/2                              7378      7062  16.8GB/s  jpg                    +4.5%
BM_UFlat/3                                92        92  2.0GB/s  jpg_200                 +0.0%
BM_UFlat/4                             15057     14974  5.9GB/s  pdf                     +0.6%
BM_UFlat/5                            204323    193140  2.0GB/s  html4                   +5.8%
BM_UFlat/6                             13282     12611  1.8GB/s  cp                      +5.3%
BM_UFlat/7                              6511      6504  1.6GB/s  c                       +0.1%
BM_UFlat/8                              2014      2030  1.7GB/s  lsp                     -0.8%
BM_UFlat/9                            775909    768336  1.3GB/s  xls                     +1.0%
BM_UFlat/10                              182       184  1043.2MB/s  xls_200              -1.1%
BM_UFlat/11                           167352    161630  901.2MB/s  txt1                  +3.5%
BM_UFlat/12                           147393    142246  842.8MB/s  txt2                  +3.6%
BM_UFlat/13                           449960    432853  944.4MB/s  txt3                  +4.0%
BM_UFlat/14                           620497    594845  775.9MB/s  txt4                  +4.3%
BM_UFlat/15                           265610    267356  1.8GB/s  bin                     -0.7%
BM_UFlat/16                              206       205  932.7MB/s  bin_200               +0.5%
BM_UFlat/17                            25561     24730  1.4GB/s  sum                     +3.4%
BM_UFlat/18                             2620      2644  1.5GB/s  man                     -0.9%
BM_UFlat/19                            45766     38589  2.9GB/s  pb                     +18.6%
BM_UFlat/20                           171107    169832  1039.5MB/s  gaviota              +0.8%
Sum of all benchmarks                3500103   3394565                                   +3.1%


Westmere:

Benchmark                          Base (ns)  New (ns)                                Improvement
-------------------------------------------------------------------------------------------------
BM_UFlat/0                             72624     71526  1.3GB/s  html                    +1.5%
BM_UFlat/1                            735821    722917  930.8MB/s  urls                  +1.8%
BM_UFlat/2                             10450     10172  11.7GB/s  jpg                    +2.7%
BM_UFlat/3                               117       117  1.6GB/s  jpg_200                 +0.0%
BM_UFlat/4                             29817     29648  3.0GB/s  pdf                     +0.6%
BM_UFlat/5                            297126    293073  1.3GB/s  html4                   +1.4%
BM_UFlat/6                             28252     27994  842.0MB/s  cp                    +0.9%
BM_UFlat/7                             12672     12391  862.1MB/s  c                     +2.3%
BM_UFlat/8                              3507      3425  1040.9MB/s  lsp                  +2.4%
BM_UFlat/9                           1004268    969395  1018.0MB/s  xls                  +3.6%
BM_UFlat/10                              233       227  844.8MB/s  xls_200               +2.6%
BM_UFlat/11                           230054    224981  647.8MB/s  txt1                  +2.3%
BM_UFlat/12                           201229    196447  610.5MB/s  txt2                  +2.4%
BM_UFlat/13                           609547    596761  685.3MB/s  txt3                  +2.1%
BM_UFlat/14                           824362    804821  573.8MB/s  txt4                  +2.4%
BM_UFlat/15                           371095    374899  1.3GB/s  bin                     -1.0%
BM_UFlat/16                              267       267  717.8MB/s  bin_200               +0.0%
BM_UFlat/17                            44623     43828  835.9MB/s  sum                   +1.8%
BM_UFlat/18                             5077      4815  841.0MB/s  man                   +5.4%
BM_UFlat/19                            74964     73210  1.5GB/s  pb                      +2.4%
BM_UFlat/20                           237987    236745  746.0MB/s  gaviota               +0.5%
Sum of all benchmarks                4794092   4697659                                   +2.1%


Istanbul:

Benchmark                          Base (ns)  New (ns)                                Improvement
-------------------------------------------------------------------------------------------------
BM_UFlat/0                             98614     96376  1020.4MB/s  html                 +2.3%
BM_UFlat/1                            963740    953241  707.2MB/s  urls                  +1.1%
BM_UFlat/2                             25042     24769  4.8GB/s  jpg                     +1.1%
BM_UFlat/3                               180       180  1065.6MB/s  jpg_200              +0.0%
BM_UFlat/4                             45942     45403  1.9GB/s  pdf                     +1.2%
BM_UFlat/5                            400135    390226  1008.2MB/s  html4                +2.5%
BM_UFlat/6                             37768     37392  631.9MB/s  cp                    +1.0%
BM_UFlat/7                             18585     18200  588.2MB/s  c                     +2.1%
BM_UFlat/8                              5751      5690  627.7MB/s  lsp                   +1.1%
BM_UFlat/9                           1543154   1542209  641.4MB/s  xls                   +0.1%
BM_UFlat/10                              381       388  494.6MB/s  xls_200               -1.8%
BM_UFlat/11                           339715    331973  440.1MB/s  txt1                  +2.3%
BM_UFlat/12                           294807    289418  415.4MB/s  txt2                  +1.9%
BM_UFlat/13                           906160    884094  463.3MB/s  txt3                  +2.5%
BM_UFlat/14                          1224221   1198435  386.1MB/s  txt4                  +2.2%
BM_UFlat/15                           516277    502923  979.5MB/s  bin                   +2.7%
BM_UFlat/16                              405       402  477.2MB/s  bin_200               +0.7%
BM_UFlat/17                            61640     60621  605.6MB/s  sum                   +1.7%
BM_UFlat/18                             7326      7383  549.5MB/s  man                   -0.8%
BM_UFlat/19                            94720     92653  1.2GB/s  pb                      +2.2%
BM_UFlat/20                           360435    346687  510.6MB/s  gaviota               +4.0%
Sum of all benchmarks                6944998   6828663                                   +1.7%