Add `bowtie diff` for diffing reports with each other

Should take as input 2 (or more?) reports and output a diff of changes between them -- i.e. tests that fail in one and not the other in an implementation, etc.

More thought required to deal with how we align tests between the two (seq may differ) as well as what to do about missing or additional implementations in one or the other, etc.

The issue does not name files, tests, or an existing CLI entry point to inspect. Start by locating how Bowtie currently reads and writes reports, then determine how reports identify tests and implementations. Done would require an agreed comparison model for two or more reports, including changed, missing, and additional tests or implementations.

Written by the indexing model from the issue text.