Switch to power of 2 tables in aav.c

Brings in somewhere in 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.