Add Event::as_raw(), Connection::resolve_event()

[wez]

In some ffi scenarios it is necessary to extract the underlying event pointer. The Event::as_raw method serves this purpose.

I couldn't implement the Raw<> trait for Event because the from_raw method requires additional context in order to perform the inverse of the as_raw method, so Event::as_raw is just a standalone method.

I added Connection::resolve_event as the inverse operation to give some symmetry.

closes: https://github.com/rust-x-bindings/rust-xcb/issues/188

[rtbo]

LGTM besides the point about doc comment. You also need to run cargo fmt

[wez]

OK! Hopefully both points are now addressed!

[rtbo]

Thank you