Support for GADT constructors with record syntax.  For now, do not try to infer a clever variance, nor a predictible ordering, for the extra arguments on synthesized types.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/constructors_with_record3@14562 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02