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/executor/nodeHash.c

@@ -755,8 +755,8 @@ ExecHashTableInsert(HashJoinTable hashtable,
  *		Compute the hash value for a tuple
  *
  * The tuple to be tested must be in either econtext->ecxt_outertuple or
- * econtext->ecxt_innertuple.  Vars in the hashkeys expressions reference
- * either OUTER or INNER.
+ * econtext->ecxt_innertuple.  Vars in the hashkeys expressions should have
+ * varno either OUTER_VAR or INNER_VAR.
  *
  * A TRUE result means the tuple's hash value has been successfully computed
  * and stored at *hashvalue.  A FALSE result means the tuple cannot match