squash closure

[hhugo]

functors with more than one arguments are compiled with nested closures.

function F (MA){
  return function(MB) { ... } }

this PR converts this pattern into a single closure function F(MA,MB) { ... }

when applying such functor, the generated code change from call_gen_1(F(MA),MB) to F(MA,MB)
This allows to eliminate more deadcode
In the following example, M1 and M2 can be removed if not used.

module type T = sig
  type t
  val x : t
end
module Make(C : T)(D : T with type t = C.t) = struct
  let a = C.x
  let b = D.x
end

module M1 = Make
    (struct type t = int let x = 10 end)
    (struct type t = int let x = 10 end)

module M2 = Make
    (struct type t = float let x = 10. end)
    (struct type t = float let x = 10. end)

[hhugo]

@vouillon, solving #56 could really improve deadcode elimination