Preliminary macOS support

[cameronelliott]

Of course, it's your call if this is useful enough or good enough to merge.

My quick testing indicates this code and the example does work. So this code might be helpful to those looking to set the affinity tags (macOS-ism) in order to get threads to run on separate cores.

The readme includes the best/only documentation I know of about the vagaries of macOS quasi thread affinities.

If you decide this is valuable enough to consider merging, let me know how/what to fix/improve.

To @elast0ny thanks for writing this, and your other Rust libs, they are great.