CI job that tests against some interesting/popular open source trees

[sourcefrog]

To discover bugs in tree and file walking, it might be good to set up an Actions matrix that tries cargo-mutants on a selection of complex Rust trees.

A few versions would be possible:

1. Just list mutants: would highlight problems reading cargo metadata, following mod statements (especially if failures become errors), and parsing source. Should be reasonably cheap.
2. Check-build each mutant. Would show something about whether we can invoke cargo correctly, but might be a poor payoff for the compute time.
3. Test a small fraction of mutants, e.g. with --shard 0/100. In general we would expect many trees to have mutants that are missed or that cause timeouts, but we could assert that it does not fail other than by finding coverage gaps.