Give a proper error if module interface has too general of a type

[DKLoehr]

Currently if we have something like

module Foo {
  fun void ('a x);
} {
  fun void ('a x) {
    int z = x + 1; // Oops, x actually has to be an int, not 'a
  }
}

The type system won't properly catch the fact that the interface type is too general.