Support for Rust Forks

[margaretjoanmiller]

Would it be possible to support Rust forks with this project such as the Crab language? https://github.com/crablang/crab

[figsoda]

toolchainOf has a root option where you can download from something other than https://static.rust-lang.org/dist if crab distributes its binaries similarly. I don't think I will directly include crab in its current state, but if there are options (for things like toolchainOf) you would like to see to better support crab, I'm happy to consider them.