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/include/nodes/nodes.h

@@ -52,6 +52,7 @@ typedef enum NodeTag
 	T_Scan,
 	T_SeqScan,
 	T_IndexScan,
+	T_IndexOnlyScan,
 	T_BitmapIndexScan,
 	T_BitmapHeapScan,
 	T_TidScan,
@@ -97,6 +98,7 @@ typedef enum NodeTag
 	T_ScanState,
 	T_SeqScanState,
 	T_IndexScanState,
+	T_IndexOnlyScanState,
 	T_BitmapIndexScanState,
 	T_BitmapHeapScanState,
 	T_TidScanState,