Move Threadpool class to separate module

[hoytak]

Move the ThreadPool class to a separate module for organization purposes, and to allow adding functionality to mix async
and non-async parallel compute.

This PR is the first step of incorporating parts of #131, which proposed a model to achieve the larger and longer term goals of:

• Allow us to isolate the async part of our codebase to only the portions that really need it.
• Abstract away the specific async runtime (now tokio) to allow us to swap it out for lighter weight runtime down the road if desired.
• Expose compute focused non-async primitives (e.g. par_for, JoinSet, JoinHandle) that we use frequently in the async world, but that could easily be done in the multithreaded sync world instead.
• Expose reliable ways to call into async routines from the non-async runtime.
• Lay the groundwork for better CTRL-C cancellations.

The XetRuntime class in this PR was the current ThreadPool class with added functions to allow spawning compute task; it was designed to allow all the current uses of tokio async primatives that would be better as compute primitives to be turned seamlessly into compute primitives.