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/spgquadtreeproc.c

@@ -30,6 +30,7 @@ spg_quad_config(PG_FUNCTION_ARGS)
 
 	cfg->prefixType = POINTOID;
 	cfg->labelType = VOIDOID;	/* we don't need node labels */
+	cfg->canReturnData = true;
 	cfg->longValuesOK = false;
 	PG_RETURN_VOID();
 }
@@ -324,6 +325,9 @@ spg_quad_leaf_consistent(PG_FUNCTION_ARGS)
 	/* all tests are exact */
 	out->recheck = false;
 
+	/* leafDatum is what it is... */
+	out->leafValue = in->leafDatum;
+
 	switch (in->strategy)
 	{
 		case RTLeftStrategyNumber: