Get rid of core_field_type, more direct representation of object types.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13489 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Update.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/extension_points@13490 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02