A simple implementation of ionice for OS X.
It can't query a process's existing ionice-ness, or set the ionice-ness of an existing process, but it can at least set the IO priority of a new process, which is all I need.
After I'd written this someone pointed out taskpolicy which
does all that this does plus more, and is bundled with the OS,
so you should probably use that instead:
https://apple.stackexchange.com/questions/423642/ionice-equivalent-for-macos