Explicitly mark fields which may be null

[msiglreith]

• Encode nullable fields of the handle types in the typesystem (Option<NonNull<_>>). The current change is not exhaustive as I'm not familiar with some platforms (e.g Web)
• Adjust docs to allow platform cases where the handles may need to be invalidated over time or further gated by platform specifics like winit/android.

Fixes #54

[Lokathor]

Windows HWND values are just *mut c_void, so they can be null. That's generally the "invalid handle" value, and all your OS calls will generally fail and such, but it's a legal bit pattern.

Other than that this seems good.

[Lokathor]

Looks good. All set on your end @msiglreith ?

[msiglreith]

fine for me 👍