* eval.c (mod_using): new method Module#using, which activates
  refinements of the specified module only in the current class or
  module definition.  [ruby-core:55273] [Feature #8481]

* test/ruby/test_refinement.rb: related test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41261 b2dd03c8-39d4-4d8f-98ff-823fe69b080e