implement fileName::clean() method

* remove repeated slashes
      /abc////def        -->   /abc/def

* remove '/./'
      /abc/def/./ghi/.   -->   /abc/def/./ghi
      abc/def/./         -->   abc/def

* remove '/../'
      /abc/def/../ghi/jkl/nmo/..   -->   /abc/ghi/jkl
      abc/../def/ghi/../jkl        -->   abc/../def/jkl

* remove trailing '/'