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

src/backend/utils/adt/array_typanalyze.c

@@ -277,7 +277,6 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 	 * worry about overflowing the initial size. Also we don't need to pay any
 	 * attention to locking and memory management.
 	 */
-	MemSet(&elem_hash_ctl, 0, sizeof(elem_hash_ctl));
 	elem_hash_ctl.keysize = sizeof(Datum);
 	elem_hash_ctl.entrysize = sizeof(TrackItem);
 	elem_hash_ctl.hash = element_hash;
@@ -289,7 +288,6 @@ compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc,
 							   HASH_ELEM | HASH_FUNCTION | HASH_COMPARE | HASH_CONTEXT);
 
 	/* hashtable for array distinct elements counts */
-	MemSet(&count_hash_ctl, 0, sizeof(count_hash_ctl));
 	count_hash_ctl.keysize = sizeof(int);
 	count_hash_ctl.entrysize = sizeof(DECountItem);
 	count_hash_ctl.hcxt = CurrentMemoryContext;