Directory diff predicate

[epage]

Based on #32, let's provide a directory predicate.

What do we care about?

• empty directories?
• .keep files in directories
• treating files as str
  • normalize line endings?
  • (with rich reporting) show a diff of files on failure?

How do we handle heterogeneous content?

• Don't
• dir-diff, filtering for the content we want, and run the dir-diff multiple times
• A lambda
• Accepting [[globs], Predicate]?

Additional resources

• https://github.com/assert-rs/dir-diff
• Expose a diffing iterator dir-diff#17
• https://github.com/cobalt-org/cobalt.rs/blob/master/tests/mod.rs#L27

[epage]

Another angle to this: sometimes we'll want to make sure two directories are the same. Sometimes we don't want our "expected" to have to contain everything, so we'd want to only diff the subset of "expected" that is in "actual".