vm.c: rb_vm_env_local_variables

* vm.c (rb_vm_env_local_variables): returns array of local
  variable name symbols in the environment by envval.
* proc.c (bind_local_variables): use rb_vm_env_local_variables.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46648 b2dd03c8-39d4-4d8f-98ff-823fe69b080e