segmented of mixed futures and values

[FrancoisChabot]

Ideally, we should be allowed to write:

Future<int, int> = async(queue, [](){
  int a;
  Future<int> b = ...;
  return segmented(a, b);
})

[FrancoisChabot]

The rules for future handler return arguments are getting convoluted. They should really be formalized, and consistently implemented, instead of being done on a feature-by-feature basis like we've been doing.,