Implement Debug for `Context`, `Surface` and `Buffer`

[madsmtm]

Builds upon #267.

[MarijnS95]

I do think having that, in addition to perhaps having some platform-specific handle (maybe even just Wayland vs X11) and format should be useful.

[madsmtm]

Would prefer if it just printed some platform-generic info like width, height, current state, etc instead of platform-specific info. Would probably be more parsable and helpful for users.

I disagree with this, I think it's often helpful to get the implementation details. For example, I might very reasonably want to debug some problem on Wayland, and then it makes sense to be able to see the state of the two buffers when I debug-print Surface.