feat: Expose other kqueue filters

[notgull]

This PR builds on the work done in smol-rs/polling#83 in order to be able to wait on kqueue events aside from file descriptor readiness. It accomplishes this by replacing the raw field in Source with a Registration field that, on kqueue platforms, accommodates all of the currently defined filters. This allows things like signals and Childs to be handled by Async<T> as well.

Draft until there is a new release of polling. I also need to test it on a BSD virtual machine.

[notgull]

The new polling version is out now! I've marked this as ready

[notgull]

Now that I look at it, maybe tying this to Async<T> isn't the best idea. The API is kind of obtuse this way, since you need to call readable(), and writable() will just silently hang forever. Maybe it would be better to have another type that just has a single ready() method.

Thoughts? Maybe we can call this type Filter<T>.

[fogti]

writable() will just hang forever

That sounds pretty bad, it should terminate either with {an I/O error or a panic} if possible...

[fogti]

imo all one-lines which can end up in a hot path (e.g. Registration::modify) should be marked #[inline]