support priority crate?

[drewcrawford]

Hi there. I wrote the priority crate to define a cross-platform, high-level coarse set of priority levels, primarily of interest to interactive-style applications. You can think of it as a much coarser version of the cross-platform ThreadPriorityValue type here. Currently, there's no way to attach those levels to threads, and so I am investigating the best path forward on that.

I wanted to take the temperature on adding support for its priority levels directly into this crate, presumably as a new enum case of ThreadPriority.

On a related and perhaps-sticking-point, macOS/iOS rely a lot on QoS classes for scheduling (pthread_attr_set_qos_class_np) which I think are not set by this crate. (And, there is a natural definition of priority's coarse levels in terms of coarse QoS values). So I also wanted to take the temperature on adding calls to pthread_attr_set_qos_class_np when setting a priority on the relevant platforms.

If there's no appetite for reviewing the PRs, feel free to close this issue and I can pursue another solution.