Park trait, ParkThread implementation and using it in LocalPool

[stbuehler]

Hi,

I think it would be good to have a Park abstration (like the one in tokio) in futures-executor (this has already been proposed and merged in #665 for the tokio-reform branch, which has been abonded in #645).

In #665 Park was renamed to Sleep, but I think Park is the better choice:

• std lib provides thread::park for a sleeping mechanism that can be cancelled ("unparked").
• tokio users already know it by that name
• it is not just a "sleep" mechanism due to the "wakeup" part

Apart from the names Sleep from #665 is basically the same as tokio Park; further changes to the API (compared to tokio):

• merge Park::park_timeout into Park::park, now taking a ParkDuration enum: most implementations I've seen used shared code anyway.
• Park::park now also includes a &mut Enter argument
• Removed Unpark trait, Park no longer has an associated Unpark type. Using futures_util::task::WakerRef instead for the result of the waker function (renamed from unpark).

I think reusing the Waker abstraction is rather obvious; but there are probably two open questions:

• Use Waker instead of WakerRef?
• Remove ParkDuration::Poll and interpret a zero duration in ParkDuration::Duration(..) as "poll only but don't yield"?

This pull request also contains a ParkThread implementation of Park (based on std::thread::park and similar to what LocalPool was using) and supports using LocalPool with custom Park implentations (defaulting to ParkThread). This way LocalPool can be combined into a "local-thread runtime" with a timer and a reactor.

References:

• tokio_executor::park
• std::thread::park

[stbuehler]

Just rebased to current master. Is there anything I can do to help moving this (and maybe other pull requests) forward?