Add an index on pg_inherits.inhparent, and use it to avoid seqscans in

 Add an index on pg_inherits.inhparent, and use it to avoid seqscans in
 find_inheritance_children().  This is a complete no-op in databases without
 any inheritance.  In databases where there are just a few entries in
 pg_inherits, it could conceivably be a small loss.  However, in databases with
 many inheritance parents, it can be a big win.