eventloop: add support for custom registrations

[jkarneges]

This makes it possible to register a callback to be called when an event::SetReadiness is invoked. The EventLoop::register_custom method is like Reactor::register_custom, except the backing event::Registration is managed internally and an event::SetReadiness is returned.

There is also some eventloop code rearrangement, using mio::Interest instead of u8 in the native API, and moving common bits into the event module. In general, we want to keep native APIs idiomatic, even if they will only be used via FFIs, and we want FFIs to be organized similarly to the native APIs they wrap.