lisp-in-types: Lisp implemented inside Rust trait system

Hello everyone! I was quite bored and decided to implement small Lisp evaluator in Rust trait system. It features `defun`, `let`,`begin`, `apply` and even delimited continuations with `call/ec`. It supports numbers from 0..8124 (can modify build.rs to generate more definitions if you want to, but requires running build with increased rustc stack size).