vm.c: constify

* vm.c (vm_make_env_each): constify pointer arguments.
  (collect_local_variables_in_iseq): ditto.
  (collect_local_variables_in_env): ditto.
  (vm_collect_local_variables_in_heap): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46647 b2dd03c8-39d4-4d8f-98ff-823fe69b080e