Implement permissions checks in Interpreter

Tracking bug for implement permissions checks in the server `Interpreter`.

Prerequisites:

* [ ] Finalise Permissions Model #392.
* [ ] Rewrite core classes to use WeakMaps #404.

To Do:

* [ ] Implement `controls()` or similar.
* [ ] Implement object mutability limits (only mutable by controllers(s)).
* [ ] Implement normal property writability limits (only writable by controller(s)).

Later:

* [ ] Implement object readability attribute? (Needed to protect `$.userDatabase` keys.)
* [ ] Implement property `readable` attribute or similar?
* [ ] Implement property `heritable` attribute or similar.