Upgrade to twox-hash 2.0

[shepmaster]

Thanks for using my crate!

Note that version 2.0 increases the MSRV to Rust 1.81. I understand if you don't want to upgrade right away, but hopefully this PR will be useful when you do.

I'm opening these PRs for the top users of twox-hash and have no current plans to become a consistent contributor to this specific repository; please feel free to rewrite the commit as appropriate for your project's requirements.