Change AAs to pow of 2 tables

This is a performance tweak for aav.c.

Citing the commit message:

Brings in somewhere in the area of 1-2% increase in speed as measured by
dmd -unittest -c std\algorithm.d
on Linux x64

This closely follows the original prime sizes by using next pow-2 of primes
and reproducing roughly the same grow factor (=4).

Takes into account that key are aligned pointers to strings
and thus need a fair bit-shake to get a decent distribution.


And lastly my measurements with avgtime, see https://github.com/jmcabo/avgtime (neat tool written in D!).
This is on linux x64. Command line is: 
avgtime -d -r 30 dmd -unittest -c std/algorithm.d

dmd_E2 
The current pull, power of 2 tables with extra hashing of keys.

------------------------
Total time (ms): 203185
Repetitions    : 30
Sample mode    : 6750 (6 ocurrences)
Median time    : 6760.67
Avg time       : 6772.83
Std dev.       : 61.7325
Minimum        : 6622.79
Maximum        : 6899.12
95% conf.int.  : [6651.83, 6893.82]  e = 120.994
99% conf.int.  : [6613.81, 6931.84]  e = 159.013
EstimatedAvg95%: [6750.74, 6794.92]  e = 22.0903
EstimatedAvg99%: [6743.79, 6801.86]  e = 29.0316

------------------------
dmd_P3 
Original with primes, but a fix for small tables of 4 items to use better index then 2 lower bits 
or it'd be just a linked list! 
Hardly helps though, could it be that tables of 4 slots ain't that common by the time the storm of lookups comes?

------------------------

Total time (ms): 204673
Repetitions    : 30
Sample mode    : 6810 (5 ocurrences)
Median time    : 6816.01
Avg time       : 6822.43
Std dev.       : 58.3656
Minimum        : 6645.7
Maximum        : 6919.68
95% conf.int.  : [6708.03, 6936.82]  e = 114.394
99% conf.int.  : [6672.09, 6972.77]  e = 150.34
EstimatedAvg95%: [6801.54, 6843.31]  e = 20.8855
EstimatedAvg99%: [6794.98, 6849.88]  e = 27.4482

------------------------
dmd 
Vanila original

------------------------
Total time (ms): 204676
Repetitions    : 30
Sample mode    : 6770 (5 ocurrences)
Median time    : 6826.8
Avg time       : 6822.55
Std dev.       : 61.4625
Minimum        : 6663.03
Maximum        : 6936.67
95% conf.int.  : [6702.08, 6943.01]  e = 120.464
99% conf.int.  : [6664.23, 6980.86]  e = 158.317
EstimatedAvg95%: [6800.55, 6844.54]  e = 21.9937
EstimatedAvg99%: [6793.64, 6851.45]  e = 28.9046

------------------------
Where does that mix function come from?