Use perfect hashing, instead of binary search, for keyword lookup.

We've been speculating for a long time that hash-based keyword lookup
ought to be faster than binary search, but up to now we hadn't found
a suitable tool for generating the hash function.  Joerg Sonnenberger
provided the inspiration, and sample code, to show us that rolling our
own generator wasn't a ridiculous idea.  Hence, do that.

The method used here requires a lookup table of approximately 4 bytes
per keyword, but that's less than what we saved in the predecessor commit
afb0d0712, so it's not a big problem.  The time savings is indeed
significant: preliminary testing suggests that the total time for raw
parsing (flex + bison phases) drops by ~20%.

Patch by me, but it owes its existence to Joerg Sonnenberger;
thanks also to John Naylor for review.

Discussion: https://postgr.es/m/20190103163340.GA15803@britannica.bec.de

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