Port to Rust

I've made a [porting to Rust](https://github.com/leo-gan/iri2020) and GitHub Pages [documentation](https://leo-gan.github.io/iri2020/). 
[Several bugs](https://leo-gan.github.io/iri2020/porting/#2-key-bug-fixes-and-numerical-corrections) were fixed in the process. 
Let me know if it makes sense to do a PR from my fork to this repo.
