Break transformCreateStmt() into multiple routines and make

Break transformCreateStmt() into multiple routines and make
transformAlterStmt() use these routines, instead of having lots of
duplicate (not to mention should-have-been-duplicate) code.
Adding a column with a CHECK constraint actually works now,
and the tests to reject unsupported DEFAULT and NOT NULL clauses
actually fire now.  ALTER TABLE ADD PRIMARY KEY works, modulo
having to have created the column(s) NOT NULL already.