Beta testing rustup.rs

Hi all. I’d like to introduce rustup, the fabulous new way to install and manage Rust toolchains. This is the successor to multirust, and began as multirust-rs by @Diggsey. Although there’s a lot of work yet to do on it, it is feature complete and I’d like to get some feedback. To try it just go to http://www.rustup.rs and follow the instructions - everything you need to know should be reachable f