Use fxhash in C-API and Python-API

[chrjabs]

Description of the Contribution

Enable the fxhash feature for the C-API and Python-API crates.
This does not only speed up hashed datastructures but also guarantee that they are deterministic.

PR Checklist

• I read and agree to CONTRIBUTING.md
• I have formatted my code with rustfmt / cargo fmt --all
• Commits are named following conventional commits
• I have added documentation for new features
• The test suite still passes on this PR
• I have added tests for new features / tests that would have caught the bug this PR fixes (please explain if not)
• If this PR contains breaking changes, it is against the next-major branch, not against main