extra::future cleanup and modernization

The `Drop` implementation was used to prevent `Future` from being implicitly copyable. Since `~fn`s are no longer copyable, this is no longer needed. I added a cfail test to make sure that this is actually the case.

I method-ized all of the `Future` creation methods and added a new one, `spawn_with`, which is similar to `task::spawn_with`.

I also got rid of some unused imports in tests.