used for const missing. do_while and do_until added.  Enumerator
  gains all of Array's abilities. Ex:
    '123'.m{|i|i*2} #=> "112233"
    '123'.pe #=> '123'.perm*' ' #=> "123 132 213 231 312 321"
  base on a patch from Darren Smith <darrenks AT ml1.net>.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@15046 b2dd03c8-39d4-4d8f-98ff-823fe69b080e