object.c: Module#singleton_class?

* object.c (rb_mod_singleton_p): new method Module#singleton_class? to
  return whether the receiver is a singleton class or not.
  [ruby-core:51087] [Feature #7609]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42449 b2dd03c8-39d4-4d8f-98ff-823fe69b080e