expose register_reader and register_writer so that manual futures can be implemented

[dvc94ch]

since rust doesn't support self referential structs there needs to be an alternative to async fn next(&self). Since self isn't owned there is no way to get it into a struct so that it can be polled.

[ghost]

How about this API, would that work for you?

impl<T> Async<T> {
    fn poll_readable(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<()>>;
    fn poll_writable(&mut self, cx: &mut Context<'_>) -> Poll<io::Result<()>>;
}

Note sure about &mut self vs self: Pin<&mut Self> though...

[dvc94ch]

yes, that would work, as it registers a waker. the problem is that at the moment there is no way to get a manual future implementation to wake up.

[dvc94ch]

Closed by #38