perf!: replace rayon with orx-parallel

[KSXGitHub]

Resolves #305

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TODO:

• Restore the --threads.

[KSXGitHub]

Experiment Results

Local measurement:

• On small directory ($THIS_PROJECT/target), the rayon implementation is 20+ times faster than the orx-parallel implementation.
• On big directory ($HOME), the orx-parallel implementation never stops. It is possible there was a deadlock of some kind.

CI: The orx-parallel implementation never stops.

Conclusion

This change is rejected.