Monkey Rust

This follows the lovely Writing an Interpreter in Go and Writing a Compiler in Go books. I did this in to practice Rust while learning more about interpreters/compilers/VMs. Many ideas following what some of the other people have already made. Though I hadn't seen the second book done in Rust yet. The compiler/vm is done, but likely not very efficient. I'd love PRs with explanations on how to handle things better.