The previous set of multipliers was not adapted for large sets of short
keys, and this new set of multipliers allows to generate perfect hash
functions for larger sets without having an impact for existing callers
of those functions, as experimentation has showed.  A future commit will
make use of that to improve the performance of unicode normalization.

All multipliers compile to shift-and-add instructions on most platforms.
This has been tested as far back as gcc 4.1 and clang 3.8.

Author: John Naylor
Reviewed-by: Mark Dilger, Michael Paquier
Discussion: https://postgr.es/m/CACPNZCt4fbJ0_bGrN5QPt34N4whv=mszM0LMVQdoa2rC9UMRXA@mail.gmail.com