dlambda - A Differentiable GADT-based interpreter

This is me playing around with two ideas:

• Writing an interpreter for a toy language that uses GADTs to (i) type-check the object langauge, and (ii) avoid dispatching when evaluating. This I unapologetically cribbed from https://github.com/goldfirere/glambda.

• Writing that interpreter so that functions defined in the object language can be differentiated by applying Ed Kmett's ad library through the interpreter.