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/include/access/hash.h

@@ -20,6 +20,7 @@
 #include "access/amapi.h"
 #include "access/itup.h"
 #include "access/sdir.h"
+#include "catalog/pg_am_d.h"
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
 #include "storage/lockdefs.h"
@@ -263,6 +264,21 @@ typedef struct HashMetaPageData
 
 typedef HashMetaPageData *HashMetaPage;
 
+typedef struct HashOptions
+{
+	int32		varlena_header_;	/* varlena header (do not touch directly!) */
+	int			fillfactor;		/* page fill factor in percent (0..100) */
+} HashOptions;
+
+#define HashGetFillFactor(relation) \
+	(AssertMacro(relation->rd_rel->relkind == RELKIND_INDEX && \
+				 relation->rd_rel->relam == HASH_AM_OID), \
+	 (relation)->rd_options ? \
+	 ((HashOptions *) (relation)->rd_options)->fillfactor :	\
+	 HASH_DEFAULT_FILLFACTOR)
+#define HashGetTargetPageUsage(relation) \
+	(BLCKSZ * HashGetFillFactor(relation) / 100)
+
 /*
  * Maximum size of a hash index item (it's okay to have only one per page)
  */