Upgrade to latest Rust

Brings us to [`9f8d2205f0518389993a9b5de6646ba8b14e5a12`](https://github.com/rust-lang/rust/tree/9f8d2205f0518389993a9b5de6646ba8b14e5a12) (June 17)

See https://github.com/mozilla/servo/pull/2677