switch to Rayon for parallel code

[birkenfeld]

and a few misc other simplifications

[abour]

Thank you for your merge request birkenfeld.

I did a performance test on my configuration with your code on a 4096*4096 image. I have the exact same result than with the actual tokio implementation (49s). I don't have any performance improvement. Did I miss something ?

[birkenfeld]

The differences I mentioned on Reddit were measured against the code checked out then. With current master I also have no seen no difference in timing. The remaining advantage is the greatly reduced amount of code.