Add test tracing, fix up comments

[carljm]

This PR does two things:

1. Add the trace feature for the test-log dependency, which I've found very useful in trying to understand the cycle-handling code. It means that I can change #[test] to #[test_log::test] above a failing test of interest, and then if I run cargo test with e.g. RUST_LOG=debug, the failing test output will include all the debug traces from that test.

2. Fix up a bunch of comments and some debug traces I ran across in my exploration that looked out of date.

Happy to split this into two PRs if that's preferred.

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[codspeed-hq]

CodSpeed Performance Report

Merging #544 will not alter performance

_{Comparing carljm:test-log (56e29b5) with master (994c988)}

Summary

✅ 1 untouched benchmarks

[MichaReiser]

You probably want to revert this change, considering that it is part of your other PR

[carljm]

I rebased this PR on that one to get the tests passing here, then noticed that since I pushed the branch to my own fork I couldn't actually set the PR base branch to the other PRs branch, so the commit from the other branch is included here. Once the other PR is merged I can rebase this one. (Or I can rebase it sooner and just let the tests fail.)

[nikomatsakis]

@carljm looks good to me but needs to be rebased. feel free to merge.

[carljm]

@nikomatsakis Rebased, but I can't merge; it says "Merging is blocked: The base branch restricts merging to authorized users."