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/nbtree/nbtutils.c

@@ -2014,7 +2014,18 @@ BTreeShmemInit(void)
 bytea *
 btoptions(Datum reloptions, bool validate)
 {
-	return default_reloptions(reloptions, validate, RELOPT_KIND_BTREE);
+	static const relopt_parse_elt tab[] = {
+		{"fillfactor", RELOPT_TYPE_INT, offsetof(BTOptions, fillfactor)},
+		{"vacuum_cleanup_index_scale_factor", RELOPT_TYPE_REAL,
+		offsetof(BTOptions, vacuum_cleanup_index_scale_factor)}
+
+	};
+
+	return (bytea *) build_reloptions(reloptions, validate,
+									  RELOPT_KIND_BTREE,
+									  sizeof(BTOptions),
+									  tab, lengthof(tab));
+
 }
 
 /*