Teach planner to optionally ignore index columns that have an equality

Teach planner to optionally ignore index columns that have an equality
constraint while determining whether the index sort order matches the
query's ORDER BY.  This for example allows an index on (x,y) to match
	... WHERE x = 42 ORDER BY y;
It only works for btree indexes, but since those are the only ones we
currently have that are ordered at all, that's good enough for now.
Per popular demand.