add a regexp crate to the Rust distribution

Links to an existing implementation, documentation and benchmarks are in the RFC. This RFC is meant to resolve issue [#3591](https://github.com/mozilla/rust/issues/3591).

I apologize in advance if I've made any amateur mistakes. I'm still fairly new to the Rust world (~1 month), so I'm sure I still have some misunderstandings about the language lurking somewhere.