Code review for domain-constraints patch. Use a new ConstraintTest node

Code review for domain-constraints patch.  Use a new ConstraintTest node
type for runtime constraint checks, instead of misusing the parse-time
Constraint node for the purpose.  Fix some damage introduced into type
coercion logic; in particular ensure that a coerced expression tree will
read out the correct result type when inspected (patch had broken some
RelabelType cases).  Enforce domain NOT NULL constraints against columns
that are omitted from an INSERT.