The universal hash table generator now uses prime table sizes instead of

The universal hash table generator now uses prime table sizes instead of
powers of two. This slows down all operations a little as we now need
to perform division instead of just AND-ing with a mask, but it allows
us to use the new hash functions in hashfunc.h which are significantly
faster than the original ones (at the expense of having bad distribution
modulo non-primes).

Also changed the limit logic to avoid rehashing when the table is already
too small or too large.