Refactor reloption handling for index AMs in-core

This reworks the reloption parsing and build of a couple of index AMs by
creating new structures for each index AM's options.  This split was
already done for BRIN, GIN and GiST (which actually has a fillfactor
parameter), but not for hash, B-tree and SPGiST which relied on
StdRdOptions due to an overlap with the default option set.

This saves a couple of bytes for rd_options in each relcache entry with
indexes making use of relation options, and brings more consistency
between all index AMs.  While on it, add a couple of AssertMacro() calls
to make sure that utility macros to grab values of reloptions are used
with the expected index AM.

Author: Nikolay Shaplov
Reviewed-by: Amit Langote, Michael Paquier, Álvaro Herrera, Dent John
Discussion: https://postgr.es/m/4127670.gFlpRb6XCm@x200m

src/backend/access/hash/hashpage.c

@@ -358,7 +358,7 @@ _hash_init(Relation rel, double num_tuples, ForkNumber forkNum)
 	data_width = sizeof(uint32);
 	item_width = MAXALIGN(sizeof(IndexTupleData)) + MAXALIGN(data_width) +
 		sizeof(ItemIdData);		/* include the line pointer */
-	ffactor = RelationGetTargetPageUsage(rel, HASH_DEFAULT_FILLFACTOR) / item_width;
+	ffactor = HashGetTargetPageUsage(rel) / item_width;
 	/* keep to a sane range */
 	if (ffactor < 10)
 		ffactor = 10;

src/backend/access/hash/hashutil.c

@@ -289,7 +289,14 @@ _hash_checkpage(Relation rel, Buffer buf, int flags)
 bytea *
 hashoptions(Datum reloptions, bool validate)
 {
-	return default_reloptions(reloptions, validate, RELOPT_KIND_HASH);
+	static const relopt_parse_elt tab[] = {
+		{"fillfactor", RELOPT_TYPE_INT, offsetof(HashOptions, fillfactor)},
+	};
+
+	return (bytea *) build_reloptions(reloptions, validate,
+									  RELOPT_KIND_HASH,
+									  sizeof(HashOptions),
+									  tab, lengthof(tab));
 }
 
 /*