Fix #364

[EliasC]

The monadic map over types did not properly recurse for the result type
of an arrow type (it called f result rather than typeMapM f result). This
caused the following program to not compile (added as test):

def fun() : (String, String) {
  ("Hello", "world")
}

class Main {
  def main() : void {
    let foo = fun : () -> (String, String);
    match foo() with
      (h, w) => print("{} {}\n", h, w)
  }
}