object.c: Object#itsef

* object.c (rb_obj_itself): new method Object#itsef.  based on the
  patch by Rafael França in [ruby-core:64156].
  [EXPERIMENTAL] this method may be renamed due to compatibilities.
  [ruby-core:44704] [Feature #6373]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47028 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-08-02

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47029 b2dd03c8-39d4-4d8f-98ff-823fe69b080e