Avoid unnecessary copies from movable futures

[tclinken]

The following code shouldn't require copy of the vector:

Future<vector<int>> produceVector(int size) {
  return vector<int>(size, 0);
}

ACTOR Future<Void> driver() {
  vector<int> v = wait(produceVector(1000));
  return Void();
}

However, the above driver does copy the vector.

[tclinken]

There is further discussion of this issue at https://forums.foundationdb.org/t/avoiding-unnecessary-copies-from-movable-futures/