Rust: integrate rust code generation into //rust/codegen

[redsun82]

Now bazel run //rust/codegen will run both the rust code generation and the python-based one from codegen.

Other changes:

• some tweaks were made by clippy
• the shuffling of the newlines in ast.py was needed to avoid the endline fixing in pre-commit to kick in
• some .. paths needed to be replaced by project_root()
• the generated rust code is now excluded from formatting

[aibaars]

This looks great, thanks!