Split the processing of INSERT/UPDATE/DELETE operations out of execMain.c.

They are now handled by a new plan node type called ModifyTable, which is
placed at the top of the plan tree.  In itself this change doesn't do much,
except perhaps make the handling of RETURNING lists and inherited UPDATEs a
tad less klugy.  But it is necessary preparation for the intended extension of
allowing RETURNING queries inside WITH.

Marko Tiikkaja