Allow unsized handle types for Arc and Rc

[a1phyr]

Lower bounds of HasRawWindowHandle impls of Arc and Rc to allow unsized types.

This allows to use eg Arc<dyn HasRawWindowHandle + Send + Sync>, which is nice to avoid generics.

However this is not done on references because it is a fundamental type, so lowering the bound is a breaking change.

[Lokathor]

if it weren't a breaking change would we also want this on references? It seems like yes.

[a1phyr]

I think that we eventually want it on references. In addition to the above advantage about avoiding generics, it costs nearly nothing, doesn't make the crate harder to maintain but makes it more consistent.