Teach planner to convert simple UNION ALL subqueries into append relations,

Teach planner to convert simple UNION ALL subqueries into append relations,
thereby sharing code with the inheritance case.  This puts the UNION-ALL-view
approach to partitioned tables on par with inheritance, so far as constraint
exclusion is concerned: it works either way.  (Still need to update the docs
to say so.)  The definition of "simple UNION ALL" is a little simpler than
I would like --- basically the union arms can only be SELECT * FROM foo
--- but it's good enough for partitioned-table cases.