Don't export the inner "sdl" module

This one might be controversial, so this is as much an RFC as anything:

Several people have gotten confused when trying to import `sdl::sdl`. It's never necessary to do that, because `rust-sdl` reexports everything under there anyway. For example, you can write `sdl::init()` instead of `sdl::sdl::init()`. So I propose just not exporting that inner module.