Update to Rust nightly

Most of the changes here are relating to namespaced enums. Arrays are also no longer implicitly passed by reference.

I've also been working on some new SDL2 audio bindings over in the rust-sdl2 project, which broke some code here. For the meanwhile, straight-up `ll::SDL_...` calls are used until something better comes around. I might send another pull request in the future to address this.