Shplait Tutorial 5 Conditionals

Summary

Syntax objects for representing expressions without evaluating them. Implementing the `subst` function to be used by `interp`. General strategy for interpreting function calls in Moe using substitution. Numbers, booleans, strings, and symbols. A look at the `fun` and application cases of `interp` and `continue`, including tracing calls on another example. The environment data structure and helper functions to implement deferred substitution. See env.rhm. Using `first`, `rest`, and `is_cons`. Distinguishing expressions from values, and treating functions as values. Representing `fun` forms and motivating the associated generalization of application forms.