Integrate with raw-window-handle crate

[ebkalderon]

Currently, this library does not provide any type-safe bindings for various kinds of window handles like we do with DevicePointer, opting to use a basic type alias instead for simplicity. Exposing such a type-safe wrapper has been a desirable goal since this library was first created. Thankfully, the existing raw-window-handle crate should fill this gap and appears to be used among several popular window libraries and graphics frameworks.

We could gate this with an off-by-default feature flag so that the extra dependency doesn't get pulled in unnecessarily.