Starting to keep attributes in the typedtree.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13440 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Represent label and constructor declarations as records in the typedtree.  Also keep return type for GADT constructors.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13441 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02