Merge the Constraint and FkConstraint node types into a single type.

This was foreseen to be a good idea long ago, but nobody had got round
to doing it.  The recent patch for deferred unique constraints made
transformConstraintAttrs() ugly enough that I decided it was time.
This change will also greatly simplify parsing of deferred CHECK constraints,
if anyone ever gets around to implementing that.

While at it, add a location field to Constraint, and use that to provide
an error cursor for some of the constraint-related error messages.