Support module aliases.
    Namely, you can write "module L = List" in signatures,
    and avoid have the compiler use directly List when you write L.
    See examples in testsuite/tests/typing-modules/aliases.ml
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/branches/module-alias@14196 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02