fix bug in Env.commit

When assigning a variable using Env.commit, the new variable was a fresh
variable and therefore we could end up pointing a non arrow variable to
it. This would break the invariant that through the DAG of the
substitution the flag can only increase.

lib/unification/Env.ml

@@ -114,7 +114,13 @@ let commit e =
  (fun v t1 t2 ->
  match t1, t2 with
  | None, None -> None
- | Some t, None | None, Some [t] -> Some t
+ | Some t, None -> Some t
+ | None, Some [t] -> (
+ match t with
+ | Type.Var _ ->
+ stack := (Type.var (tyenv e) v, t) :: !stack; (* TODO: we will create a new variable because of this. We need to work to create less variables in general. *)
+ None
+ | _ -> Some t)
  | None, Some l -> Some (Type.tuple (tyenv e) (Type.NSet.of_list l))
  | Some t1, Some l ->
  (* TODO: Should we do something more clever here? Like look for the repr or