Rayon support

Hi! How to use shuttle to test packages based on Rayon? My understanding is that shuttle has no direct support for Rayon primitives, so we need to change source code of Rayon. Is it true? Or is there any existing code base of Rayon adapted for shuttle?

Also, it will be great to add direct Rayon support.

Start by reviewing Shuttle's current support for concurrent Rust primitives and how packages based on Rayon integrate with it. Determine whether Rayon requires source changes or an existing adapted codebase, then define what direct Rayon support would need to cover.

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