Shorten syntax for functor signatures

```
(* Optional naming of paramater 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 (_:S) (Y:S) -> ...
```
Pretty-print the shortened forms of functor signatures.
Add tests for the shortened functor signatures