Implement context simplification for subclasses

[wbbradley]

class F x {
  fn f(x) Int
}

class G x {
  has F
  fn g(x) Int
}

fn h(y) => g(y)

Should result in scheme (∀ a . a -> Int) | [G a] not (∀ a . a -> Int) | [F a, G a] because G a implies F a.

[wbbradley]

This becomes more important if I decide to add default instance selection to the language. Currently it's not an issue.