Undo not-so-hot decision to postpone insertion of default values into

Undo not-so-hot decision to postpone insertion of default values into
INSERT statements to the planner.  Taking it out of the parser was right
(so that defaults don't get into stored rules), but it has to happen
before rewrite rule expansion, else references to NEW.field behave
incorrectly.  Accordingly, add a step to the rewriter to insert defaults
just before rewrite-rule expansion.