ThreadPoolScheduler

`DexThreadPoolScheduler` is a [class@Dex.

Scheduler] that will dispatch work items and fibers to sub-schedulers on a specific operating system thread.

[class@Dex.Fiber] will never migrate from the thread they are created on to reduce chances of safety issues involved in tracking state between CPU.

New work items are placed into a global work queue and then dispatched efficiently to a single thread pool worker using a specialized async semaphore. On modern Linux using io_uring, this wakes up a single worker thread and therefore is not subject to "thundering herd" common with global work queues.

When a worker creates a new work item, it is placed into a work stealing queue owned by the thread. Other worker threads may steal work items when they have exhausted their own work queue.