don't use rayon on files where it will be slower

[rot13maxi]

when a user asks for turbo mode, first stat the file and make sure its over 128k, if its not just hash normally. According to the blake3 docs, rayon is slower for files smaller than this.

[casey]

I'm actually wondering if we should do this! update_mmap_rayon calls update_mmap, which says:

In those cases, this method will fall back to standard file IO. The heuristic for whether to use memory mapping is currently very simple (file size >= 16 KiB), and it might change at any time.

So we'll already fall back to not using rayon if the filesize is below 16 KiB, so it seems like it might not be a huge win to hard-code the case between 16 KiB and 128 KiB, and makes it slightly less predictable for the user, since without implementing our own heuristics, the different modes map directly to the different functions in the blake3 crate.