proc.c: Binding#local_variables

* proc.c (bind_local_variables): allowing binding to list its
  local variables.  patch by Jack Danger Canty <jackdanger AT
  squareup.com> at [ruby-core:56543].  [Feature #8773]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44392 b2dd03c8-39d4-4d8f-98ff-823fe69b080e