update with new rust version

[chenyukang]

A friend recently asked me for recommendations on a good project to begin learning Rust. I recalled that I started with a simple scheme interpreter years ago (https://github.com/chenyukang/rust-scm).

Later, I came across this project which I think might be a more suitable option. It’s well-tested and comes with an accompanying online book in Go, as well as your excellent blog post on the learning experience. I took a little time to review the code and updated it to address a few outdated Rust conventions.

thanks!