Avoid rayon usage when running in single-threaded mode.

[TomAFrench]

We currently use rayon unconditionally but we should be able to avoid this entirely if we're running with a single thread.

[asterite]

I thought about this, but I think we also increase the stack size of the threads we spawn. So I think we need to always span at least one thread to be able to control its stack size.

[TomAFrench]

I think that's only the case as rayon threads are by default given a smaller stack size compared to the main thread.