Allow [@mel.as ...] in module definitions

[anmonteiro]

(* x.ml *)

module Object = struct end

$ melc -ppx melppx x.ml
// Generated by Melange
'use strict';


var $$Object = {};

exports.$$Object = $$Object;
/* No side effect */

should be possible to use [@mel.as ..] like it is for let bindings

[anmonteiro]

fixed in #879