Re-evaluate parallelization (Rayon)

[jjhbw]

Seems lik the --no-default-features flag (which disables Rayon multiprocessing) actually leads to a serious speedup of detect(), from ~1000 ms to ~700 ms on the scientists.jpg image. Likely due to shaving off threading overhead. This is on a noisy dual core 2018 MBP, so YMMV.

I think larger images may benefit more from parallel processing, let's test that first. We may need to re-evaluate whether we want parallel processing on by default.