Shorten syntax for functor signatures (patch by Thomas Gazagnaire)
    
    ```
    (* Optional naming of parameter implementation *)
    module type X = functor (X:S) -> ...
    module type X = functor (_:S) -> ...
    
    (* shortening of functor module signatures *)
    module type F = functor (X:S) -> functor (Y:S) -> ...
    module type F = functor (X:S) (Y:S) -> ...
    ```
    
    For consistency reasons, this commits also add shortening of functor implementations:
    
    ```
    (* shortening of functor implementations *)
    module F = functor (X:S) -> functor (Y:S) -> ...
    module F = functor (X:S) (Y:S) -> ...
    ```
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14474 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02