Feature request: suppress/abbreviate module type output

[dlesbre]

Maybe related to #282.

When defining module in an interactive session, OCaml tends to print the full expended signature:

# module M = Map.Make(Int);;
module M :
  sig
    (* Insert >50 lines here *)
  end

Having this in your readme or documentation is basically never desired behavior. Having a way to shorten it or use named modules would be nice:

• option one: allow a abbreviated version (perhaps with a simple label to toggle on off).
  With this option, this should not generate any error:

  <!-- $MDX abbrev -->
  ```ocaml
  # module M = Map.Make(Int);;
  module M : sig ... end 
  ```

• option two: allow subtyping (probably much harder to do).
  with this option this should also check that M can be given the supplied type

  <!-- $MDX module-subtypes -->
  ```ocaml
  # module M = Map.Make(Int);;
  module M <: Map.S with type key = int
  ```

  (a common hack to check this it to add a module _ : Map.S with type key = int = M after the definition of M).

I should mention that there is a way to avoid this problem: using non-toplevel code blocks (one that don't start with #), since they don't produce any output. (Also, maybe mention these exist in the readme? Finding them took a while...).

```ocaml
module M = Map.Make(Int)
```

However, it means you sometimes have to alternate code blocks frequently between toplevel/non-toplevel, which can be annoying and look a bit weird in the output.