Write an interpreter for the simply typed lambda calculus

Let's follow up on our interpreters for the untyped lambda calculus with interpreters for the simply typed lambda calculus.

As Tikhon pointed out, the interpreter part of this project can actually be identical to the interpreter for the untyped lambda calculus, since typechecking is performed before interpretation. But you might find the language you're working on more interesting if you beef it up with a few built-in data types.