proc.c: method_super_method

* proc.c (method_super_method): new method Method#super_method,
  which returns a method object of the method to be called by
  `super` in the receiver method object.
  [ruby-core:62202] [Feature #9781]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46964 b2dd03c8-39d4-4d8f-98ff-823fe69b080e