Make the `WorkStealingThreadPool` be an `IORuntime`

The inheritance is yet to be determined properly.

1. The WSTP is a compute pool.
2. The WSTP has good support for blocking, and we can cache the threads that were used for blocking, similar to a cached thread pool.
3. We already want to give the WSTP scheduler capabilities.
4. This will solve the situation where creating a WSTP needs a forward reference to the runtime it is in.

I will work on this in the coming months.