Expose the Events structure instead of using Vec<Event>

[notgull]

My main complaint with the polling API is the fact that it doesn't expose sys::Events; instead, it reads events from the sys::Events into a Vec<Event>. In practice, this usually means that:

1. System events are read to Events.
2. poller events are read out of the Events.
3. poller events are appended to a Vec<Event>.
4. Downstream, the poller events are then drained out of that Vec<Event> and used to react to events.

Steps 1 and 2 are necessary, but steps 3 and 4 feel unnecessary. It would be nice if the Events structure was exposed and given a real API. Is there any particular reason why polling was designed to use Vec<Event> in the first place?

This would be a breaking change, but there's already breaking changes on the horizon (#38) so we should probably at least plan for this.