Lazy evaluation

[jespercockx]

Our evaluator is already set up with the possibility of lazy evaluation, all we'd need to do is the following:

• [ ] Keep track of which values in the environment have already been evaluated
• [ ] Add a new 'update' frame for updating a value in the environment with the current focus term
• [ ] Push an update frame to the stack whenever we start evaluating an unevaluated variable.