iterative, recursive file finder

 Path::Iterator::Rule iterates over files and directories
 to identify ones matching a user-defined set of rules.
 The API is based heavily on File::Find::Rule,
 but with more explicit distinction between matching rules
 and options that influence how directories are searched.
 A Path::Iterator::Rule object is a collection of rules (match criteria)
 with methods to add additional criteria.
 Options that control directory traversal are given as arguments
 to the method that generates an iterator.
 .
 Here is a summary of features
 for comparison to other file finding modules:
  * provides many "helper" methods for specifying rules
  * offers (lazy) iterator and flattened list interfaces
  * custom rules implemented with callbacks
  * breadth-first (default) or pre- or post-order depth-first searching
  * follows symlinks (by default, but can be disabled)
  * directories visited only once (no infinite loop; can be disabled)
  * doesn't chdir during operation
  * provides an API for extensions
 .
 As a convenience, the PIR module is an empty subclass of this one
 that is less arduous to type for one-liners.