Improve hash_create()'s API for some added robustness.

 Improve hash_create()'s API for some added robustness.
 
 Invent a new flag bit HASH_STRINGS to specify C-string hashing, which
 was formerly the default; and add assertions insisting that exactly
 one of the bits HASH_STRINGS, HASH_BLOBS, and HASH_FUNCTION be set.
 This is in hopes of preventing recurrences of the type of oversight
 fixed in commit a1b8aa1e4 (i.e., mistakenly omitting HASH_BLOBS).
 
 Also, when HASH_STRINGS is specified, insist that the keysize be
 more than 8 bytes.  This is a heuristic, but it should catch
 accidental use of HASH_STRINGS for integer or pointer keys.
 (Nearly all existing use-cases set the keysize to NAMEDATALEN or
 more, so there's little reason to think this restriction should
 be problematic.)
 
 Tweak hash_create() to insist that the HASH_ELEM flag be set, and
 remove the defaults it had for keysize and entrysize.  Since those
 defaults were undocumented and basically useless, no callers
 omitted HASH_ELEM anyway.
 
 Also, remove memset's zeroing the HASHCTL parameter struct from
 those callers that had one.  This has never been really necessary,
 and while it wasn't a bad coding convention it was confusing that
 some callers did it and some did not.  We might as well save a few
 cycles by standardizing on "not".
 
 Also improve the documentation for hash_create().
 
 In passing, improve reinit.c's usage of a hash table by storing
 the key as a binary Oid rather than a string; and, since that's
 a temporary hash table, allocate it in CurrentMemoryContext for
 neatness.
 
 Discussion: https://postgr.es/m/590625.1607878171@sss.pgh.pa.us