Shplait Lambda 5 Closures

Summary

Looking at the evaluation of `fun` forms and the representing the result as a Encoding pairs using just functions. Implementing the `subst` function to be used by `interp`. Distinguishing expressions from values, and treating functions as values. Updating `interp` to support `fun` expressions. The λ-calculus is an even simpler language than Moe. It's a Turing-complete language that was invented by Alonzo Church in the ... Parsing for Moe with `fun` expressions and the revised `Exp` datatype. Representing `fun` forms and motivating the associated generalization of application forms. Local bindings via `let` can be implemented by making Moe's `parse` convert it into an immediately applied `fun` form. Type rules in the traditional, math-ish notation. Lexical scope lets us predict, before running a program, where in the environment each variable will appear.