proc.c: Binding#receiver

* proc.c (bind_receiver): new method to return the bound receiver
  of the binding object.  [ruby-dev:47613] [Feature #8779]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46646 b2dd03c8-39d4-4d8f-98ff-823fe69b080e