Ideas

Some ideas:

1. Add builder API for ThreadGroup (with wrapping?)
2. Add builder API for ThreadFactory (with wrapping?)
3. Add builder API for ThreadPoolExecutorService and alike
4. Promises.fromOptional(Optional<T>) -- to completed (success or failure) future
5. Promise.toOptional() -> Promise<Optional<T>> ???
6. Implements Iterable for completed promise? Implements Stream for completed promise?
7. Promise.thenFilter
8. Some composition futures like https://github.com/spotify/trickle (if ever necessary)