Both epoll and kqueue support listening for other epoll/kqueue file descriptors.

They become readable when an event in the inner epoll/kqueue is ready.

We can expose this feature by making Poll an event::Source.

I gated this feature behind os-ext as this was required to use IoSource.

Additionally, thanks to Rust's borrow checker, there can be no cycles in the listening graph i.e. this will not compile:

let mut poll = Poll::new().unwrap();

poll.registry().register(&mut poll, Token(0), Interest::READABLE).unwrap();