Rewrote in Rust, set up gravpack and automatic builds

With this latest version, I’ve completely rewritten all the GDScript bits in Rust. It’s working pretty well so far, and was a pleasant transition.

What wasn’t a pleasant change was getting CD working correctly. That took many hours, but now, I should be able to get builds automatically uploaded on supported platforms whenever I push a change. Can’t quite do this on MacOS yet, because cross-compiling for MacOS isn’t yet functional. Maybe some day I’ll get it set up, but Macs are a pain to work with.