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

src/include/parser/kwlist.h

@@ -21,8 +21,7 @@
 /*
  * List of keyword (name, token-value, category) entries.
  *
- * !!WARNING!!: This list must be sorted by ASCII name, because binary
- *		 search is used to locate entries.
+ * Note: gen_keywordlist.pl requires the entries to appear in ASCII order.
  */
 
 /* name, value, category */