Support long-lived values that are not bound to &Env (GlobalValue)

[ubolonton]

• Module code can store values for later use, e.g. in background threads.
• Frequently-used symbols won't require repeated intern calls into the Lisp runtime. We can have emacs::sym::nil, emacs::sym::t. Modules will also be able to declare symbols they often use.
• Error handling will not need unsafe (even though the safety requirement is almost trivial currently).

The biggest problem with this is that free_global_ref() requires an emacs_env, thus cannot be used directly in GlobalValue::drop() (this is likely a mis-design in emacs-module (BEAM's enif_release_resource doesn't need an env, for example)). There are 2 potential ways to solve this:

• Add fn free(self, env: &Env), and just leak on normal drops. They are supposed to be long-lived anyway.
• Add a "mini-GC" by globally tracking to-be-free values, and freeing them in Env::drop(), once in a while.

[ubolonton]

This was added in release 0.13.0, but has not been stabilized/documented yet.