src: use stack-allocated Environment instances

Makes it easier to reason about the lifetime of the Environment object.

PR-URL: https://github.com/nodejs/node/pull/7090
Refs: https://github.com/nodejs/node/pull/7082


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>