Rearrange the implementation of index-only scans.

This commit changes index-only scans so that data is read directly from the
index tuple without first generating a faux heap tuple.  The only immediate
benefit is that indexes on system columns (such as OID) can be used in
index-only scans, but this is necessary infrastructure if we are ever to
support index-only scans on expression indexes.  The executor is now ready
for that, though the planner still needs substantial work to recognize
the possibility.

To do this, Vars in index-only plan nodes have to refer to index columns
not heap columns.  I introduced a new special varno, INDEX_VAR, to mark
such Vars to avoid confusion.  (In passing, this commit renames the two
existing special varnos to OUTER_VAR and INNER_VAR.)  This allows
ruleutils.c to handle them with logic similar to what we use for subplan
reference Vars.

Since index-only scans are now fundamentally different from regular
indexscans so far as their expression subtrees are concerned, I also chose
to change them to have their own plan node type (and hence, their own
executor source file).

src/backend/optimizer/plan/subselect.c

@@ -1974,6 +1974,18 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 			context.paramids = bms_add_members(context.paramids, scan_params);
 			break;
 
+		case T_IndexOnlyScan:
+			finalize_primnode((Node *) ((IndexOnlyScan *) plan)->indexqual,
+							  &context);
+			finalize_primnode((Node *) ((IndexOnlyScan *) plan)->indexorderby,
+							  &context);
+
+			/*
+			 * we need not look at indextlist, since it cannot contain Params.
+			 */
+			context.paramids = bms_add_members(context.paramids, scan_params);
+			break;
+
 		case T_BitmapIndexScan:
 			finalize_primnode((Node *) ((BitmapIndexScan *) plan)->indexqual,
 							  &context);