Flesh out LocalPoolHandle

[Darksonn]

The LocalPoolHandle type is currently very simple. It would be nice to see the following functionality added:

1. Spawning a future on a specific thread.
2. Spawning a copy of a future on every thread.
3. A function that returns the number of threads.

The documentation should also be improved to explain that you can use spawn_local, and that this will spawn the given future on the same thread. It should also be mentioned that the handle can be cloned.

[b-naber]

I can try to work on this.