Codegen Rust comments

[emilk]

What

Reading the generated Rust code is… difficult.
Trying to map some generated piece of code to the code that generated that is likewise difficult.
With this PR we can start sprinkle in comments where it makes sense.

This is what I do when I wake up at 4am and can't fall back to sleep. Code quality might reflect that.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.