Add mkpath()

Author: lizmat

S16-io.pod

@@ -258,6 +258,7 @@ An overview:
 
   mkdir($dir)                   # create a directory
   rmdir($dir)                   # remove a directory
+  mkpath($path)                 # create directory and parents as appropriate
 
   chdir($dir)                   # set $*CWD
   temp $*CWD = chdir($dir)      # set $*CWD for the current scope