crate_universe: Rewrite generator in starlark

This is just an idea, I'd really appreciate some feedback and ideas.

Is it conceivably possible to rewrite the generator in pure starlark? This would remove the bootstrapping problem of distributing generator binaries or hackily building the generator with the native cargo toolchain.

https://github.com/bazelbuild/rules_rust/blob/59fab4e79f62bfa13551ac851a40696c24c0c3a4/crate_universe/deps_bootstrap.bzl#L9

We're using a custom generator to work around some issues, but this makes the analysis phase almost a minute slower as repository rules are not remotely cacheable.

```
Finished release [optimized] target(s) in 38.77s
```

Failing a full rewrite, some more ideas:
- Rewrite only some functionality in starlark.
- Move some functionality out of the repository rule and into just a normal rule. This could be achieved with a macro. The normal rule could then check if repinning is required, whilst allowing the repository rule to generate the build files it needs.

Thanks for taking the time to read this :) Please don't hesitate to ask any questions.