Move Hashbrown to be optional

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Description

Moves Hashbrown as optional dependency to allow compiling on systems without atomics. Long story short - unfortunately just making Hashbrown's ahash optional on the workspace will not solve everything as it removes HashMap::new() too, that is used in few places. Here is a variant that moves multivariate commits under feature flag, that were the only part that depends on the Hashbrown. At the moment benchmarks in poly part are broken as they depend on multivariate part. I believe it's possible to fix one way or another, just need to know which one.

Another option would be to try to make hash use explicit (and not use ahash), will be posted in alternative PR

Fixes #812

• 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