Draft prototype using traits

[hecrj]

As discussed in #16, an approach using traits instead of non-exhaustive enums/structs. I also added an example showcasing how a create_surface implementation for MacOS and Linux would look with this approach.

This is probably a controversial change. I think we should dismiss this idea if there is no clear consensus.

[Osspial]

cc @kvark @Ralith

Looking over the changes for this, I've got a couple points of feedback:

1. The biggest downside to this is that it makes the documentation significantly less readable. There are probably ways we can compensate for that, but I don't think we can get rustdoc to render this as intuitively as we have set up now.
2. I can see the current solution for the X11/Wayland backend on Unix being difficult to implement, since it requires auxiliary types for the HasX11Handle and HasWayland handle traits. That won't be a problem for Winit, but I assume it'll be more of a hassle for SDL wrappers.

One solution that I believe would largely keep the benefits of both the current design and this redesign would be to expose all the window-handle-getters in the top-level trait, then conditionally compile each of those functions so that they're only available on their target platform. I'd write up a sketch of that design, but I just looked back through the original thread and... it looks like that's pretty much the original design I proposed. Welp.

[Osspial]

I'm going to close this, since it's not an unambiguously good change to the codebase. Thank you for pulling this together, though!