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/nodes/print.c

@@ -320,14 +320,18 @@ print_expr(Node *expr, List *rtable)
 
 		switch (var->varno)
 		{
-			case INNER:
+			case INNER_VAR:
 				relname = "INNER";
 				attname = "?";
 				break;
-			case OUTER:
+			case OUTER_VAR:
 				relname = "OUTER";
 				attname = "?";
 				break;
+			case INDEX_VAR:
+				relname = "INDEX";
+				attname = "?";
+				break;
 			default:
 				{
 					RangeTblEntry *rte;