Consider using rayon to parallelize this

Hey, super nice tool. I'm now using this with fzf.vim for great effect. However, after some benchmarking, I can't help but wonder whether perhaps we'd get an even faster result by parallelizing the sorting. In a benchmark via hyperfine, it takes my laptop about 3s to order a list of 2.2m files (all files on my computer). That's not bad at all but still perceivable. Perhaps we can get it down to 0.8s which would be great. Perhaps something like [par_sort_by](https://docs.rs/rayon/1.3.1/rayon/slice/trait.ParallelSliceMut.html#method.par_sort_by) could be used with a custom criterion?