Convert `gen.py` to `build.rs`?

[hoijui]

Seems like that should be fairly easy, and it would remove the need for python, it would:

1. ensure that classes.rs is always the latest,
2. it would not require manual calling of the generation code before a build anymore,
3. and classes.rs would not have to be git-tracked anymore.

I bet you have better things to do, and so have I, but I though I'd put it out there as an idea anyway.