A library for matching files using patterns such as "src/**/*.png" for
all .png files recursively under the src directory. Features:

* All matching is O(n). Most functions precompute some information
  given only one argument.

* See "System.FilePattern" and '?==' simple matching and semantics.

* Use 'match' and 'substitute' to extract suitable strings from the *
  and ** matches, and substitute them back into other patterns.

* Use 'step' and 'matchMany' to perform bulk matching of many patterns
  against many paths simultaneously.

* Use "System.FilePattern.Directory" to perform optimised directory
  traverals using patterns.