Deduce equality constraints that are implied by transitivity of

mergejoinable qual clauses, and add them to the query quals.  For
example, WHERE a = b AND b = c will cause us to add AND a = c.
This is necessary to ensure that it's safe to use these variables
as interchangeable sort keys, which is something 7.0 knows how to do.
Should provide a useful improvement in planning ability, too.