feat: Auto-recompile Rust side when source changes

[ethanuppal]

Closes #165

• Throwing in an include_str doesn't work if you want to build from within marlin (for instance, having it automatically call out to swim build) because the file will not exist. It also means the file won't be there until you run a swim build` or similar in the first place.
• Not so sure about my relative path check. However I think it is reasonable given we require all non-Verilog frontends provide absolute paths from their API wrappers.