Update rust-sdl2_mixer to depend on sdl2-sys for low level SDL2 types, and move ffi.rs into sdl2_mixer-sys to conform to established cargo conventions

rust-sdl2 no longer directly exports the unsafe ("ll") sdl2 declarations, as these are now provided by the sdl2-sys library. Moreover, rust-sdl2_mixer needs to be updated to follow suit, as this API separation is part of established cargo conventions.