add a git pre-commit hook that ensures no structs or enums outside types.rs

[etosch]

We want to enforce that types all live in <gamename>/src/types.rs. Need to write a small program to do this that we can run as a pre-commit hook.

This is a good issue for non-experts because it doesn't require knowledge of much of rust, or Toybox. A very rudimentary version could just grep through the file for structs and enums. This could put restrictions on comments. A more sophisticated version might actually parse in the rust file and to determine if we have structs of enums.