Use `rust-sdl2` and link to `SDL2_ttf`
Refactor library structure

* AdAdds a `tests` crate which is only compiled for `rustc --test`
* AdAdds a `make test` target which will compile & run the tests crate.
* Moves `ffi` to its own crate to separate wrapper from bindings.
Add `rust-sdl2` submodule.

* The `make deps` target will init, fetch, and build `rust-sdl2` for you.
* Resulting lib is placed in `lib/` which is incl. by all other targets.