Replace Ahash with Fnv for Hashbrown

[shamatar]

Alternative to #824 , will fix #812 too for systems without atomics. Fnv provides no key randomization, but it shouldn't be required for commitment scheme.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[Pratyush]

How about we use SipHash for non-atomic platforms, and ahash elsewhere?

[shamatar]

To be honest I'm not sure from top of my head if I can make cargo dependency optional based on whether a platform has atomics. Will try

[shamatar]

Another question is whether or not try to include aHash with or without randomization if it's done as external dependency instead of default hash in hashbrown. Hashbrown itself doesn't have and option to continue aHash's features, but maybe it can be imported independently and without randomization is may have no atomics by itself

[Pratyush]

To be honest I'm not sure from top of my head if I can make cargo dependency optional based on whether a platform has atomics. Will try

I think it's possible like so: https://github.com/PyO3/pyo3/blob/fe79f548174eb8108813f202bb0df9428ddfd806/Cargo.toml#L48

[shamatar]

restored features (inlines) on hashbrown + ahash is used on platforms with atomics