Return types are not checked properly

[mbenke]

Consider the following (admittedly silly) function:

  function even (n) -> Bool {
    match n {
    | Zero => return 1; return True;
    | Succ[m] => return 0; return False;
    };
  }

it typechecks ok, though it probably should not.

[rodrigogribeiro]

Fixed on last commit on main.