Dead code detection

vulture does a decent job at this. clippy does an amazing job at this.

We don't want to emit this for 'public' methods. I don't know if we just want to check `__all__`, or if we want to be more lenient, personally I think an option to say I don't want to export anything, so the rule is very pedantic would be great, and the other side of that is maybe only complaining about 'private' (prefixed with '_') methods that are unused.

No files, tests, or entry points are named in the issue, so first locate the existing diagnostics for unused methods and review how public methods and __all__ are represented. Done should include a decided policy or option for public/exported methods and diagnostics for the intended unused private methods, with tests covering the chosen behavior.

Written by the indexing model from the issue text.