Remove extras

Remove extras/ and use a git submodule instead. Also, in the near future we can replace the submodule with nothing at all, and make it mandatory to call `pip install pokemontools`. Keeping the submodule around is also an okay thing to do.

The git submodule points to the git repository over here:
https://github.com/kanzure/pokemon-reverse-engineering-tools

So far, everything works and the game compiles. But I may have missed a few things, so please double check my work.