vm_eval.c: rb_current_receiver

* vm_eval.c (rb_current_receiver): new function to return the
  receiver in the current control frame.  [Feature #10195]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48462 b2dd03c8-39d4-4d8f-98ff-823fe69b080e