Clean up OPAM directories if requested when running `topgrade`

[Tyrubias]

OPAM has a built-in clean command that automatically removes download caches, logs, and cleans the current OPAM switch. We should call opam clean when the cleanup flag is set.

Standards checklist:

• [x] The PR title is descriptive.
• [x] The code compiles (cargo build)
• [x] The code passes rustfmt (cargo fmt)
• [x] The code passes clippy (cargo clippy)
• [x] The code passes tests (cargo test)
• [x] Optional: I have tested the code myself
  • [x] I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the means to test it, please tag this person here.

[s34m]

Merged at new fork because of inactivity by maintainer.