implement UVWork to combat resource problems and enhance performance when no default executor is being utilized.

[Vizonex]

What do these changes do?

I had an idea about using libuv's thread-pool as a backup mechanism to help eliminate a few resource heavier elements such as the ThreadPoolExecutor that was originally utilized for the functions that I have now changed making it possible to do a few faster things when needing to lazily use a function like asyncio.to_thread(...) There are a few caveats such as not being able to control how many threads are spun up but this new implementation should be very minimal on resources and very beneficial.

I added in a test for this function to make sure it runs correctly but now there should no longer be a chain of 3 items before reaching the functions
Orgininally it was

asyncio.Future -> concurrent.futures.Future -> _WorkItem

Now it is setup this way making it a lot faster to reach.

asyncio.Future -> UVWork

Are there changes in behavior for the user?

Is it a substantial burden for the maintainers to support this?

Related issue number

fixes #111

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes