Teach SP-GiST to do index-only scans.

Operator classes can specify whether or not they support this; this
preserves the flexibility to use lossy representations within an index.

In passing, move constant data about a given index into the rd_amcache
cache area, instead of doing fresh lookups each time we start an index
operation.  This is mainly to try to make sure that spgcanreturn() has
insignificant cost; I still don't have any proof that it matters for
actual index accesses.  Also, get rid of useless copying of FmgrInfo
pointers; we can perfectly well use the relcache's versions in-place.

src/backend/access/spgist/spgkdtreeproc.c

@@ -30,6 +30,7 @@ spg_kd_config(PG_FUNCTION_ARGS)
 
 	cfg->prefixType = FLOAT8OID;
 	cfg->labelType = VOIDOID;	/* we don't need node labels */
+	cfg->canReturnData = true;
 	cfg->longValuesOK = false;
 	PG_RETURN_VOID();
 }