Executor

This is a simple executor. There is a single Arc<Executor> type that can be shared between many worker threads.

Each worker will continuously run executor.poll_once() which will

1. Pop a task from the shared task queue
   • Parking the thread if no tasks are available
2. Poll that task

The task will be polled with a context that will reference that task (using a Mutex<Option<Task>>), and a copy of the executor.

Calling wake on this context will then call executor.wake(task). If there are threads sleeping, one will be woken up.