TODO for Path::Class::Rule

- improved method argument validation
- what do we do about search directories that don't exist?
- should directories be tested or automatically expanded?
- custom sorting?
- normalize paths to unix slashes?
- memoize Path::Class::stat for duration of test()?
- cache File::stat in state structure; reimplement all filetests
  to use cached stat and Fcntl constants instead of -X operators

# rule helper ideas
- path (full path matching, not just basename)
- dotfiles
- backups (*~, etc.)


