Update crossbeam-utils requirement from 0.7 to 0.8
Updates the requirements on [crossbeam-utils](https://github.com/crossbeam-rs/crossbeam) to permit the latest version.
- [Release notes](https://github.com/crossbeam-rs/crossbeam/releases)
- [Changelog](https://github.com/crossbeam-rs/crossbeam/blob/master/CHANGELOG.md)
- [Commits](https://github.com/crossbeam-rs/crossbeam/compare/crossbeam-utils-0.7.0...crossbeam-utils-0.8.0)