Instead of supposing (wrongly, in the general case) that the rowtype

Instead of supposing (wrongly, in the general case) that the rowtype
of an inheritance child table is binary-compatible with the rowtype of
its parent, invent an expression node type that does the conversion
correctly.  Fixes the new bug exhibited by Kris Shannon as well as a
lot of old bugs that would only show up when using multiple inheritance
or after altering the parent table.