[ortac-monolith] generation of Monolith.spec for function arguments that are not core_type is broken

[n-osborne]

val f : (int * int * int) -> bool
(*@ b = f x *)

will give:

(* ... *)
module G = struct  end
module P = struct  end
module S = struct  end
let () = let spec = S.tuple3 ^!> bool in declare "f is Ok" spec R.f C.f
let () = let fuel = 100 in main fuel

Where the generation of the test declaration expect to find the definition of a 3-uplet of int in the module S, which is empty.

As the limitation of tuples of length 2 comes from monolith itself, we could keep this limitation.