Replace fxhash with rustc-hash

[paolobarbolini]

The fxhash crate hasn't been maintained in the last 6 years. This migrates to rustc-hash, which is much more popular crate developed by the Rust project and used by the Rust compiler.

[mrobinson]

@paolobarbolini Thanks for the contribution.

This crate is a downstream version of Gecko's style. We ask that non-Servo-specific changes be submitted upstream: https://searchfox.org/mozilla-central/source/servo/components.

[nicoburns]

There's a guide to submitting patches to Gecko here: https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html

However, I would note that "fxhash" stands for Firefox Hash so I'm not sure how open they would be to this change.