Simplification & refactor of (parts of) the runtime

* Proper lowercasing for internal stuff
* *Environment => *_stash
* ExecutionContext => _scope
* Simpler & shallower call/construct mechanics
* Remove unnecessary fields & methods
* Better scoping (no more stack): []*_scope => _scope.outer
* Some speed improvements

In preparation for #66
Minor cloning adjustments

Mostly cosmetic.