Fut<A> should have covariant subtyping (right now it is invariant)

[bezirg]

Example code that does not typecheck with habs:

module Test4;

data List_<A> = Nil_
              | Cons_(A,List_<A>);

interface I {
  I m();
}

interface I_ extends I {
  I_ m_();
}

class C implements I {
  I m() {
    return this;
  }
}

class C_ implements I_ {
 I m() {
   return this;
 }
 I_ m_() {
   return this;
 }
}

{
 I o = new local C();
 I_ o_ = new local C_();

 Fut<I_> f_ = o_!m_();

 Fut<I> f = f_; // covariant Future breaks

 // or Fut<I_> f_ = o_!m_();
 
}