"WIP" only because I still need to add documentation.

Just an informative "by the way" comment that might be interesting to think about, but doesn't affect this PR much.

Here's a quick comparison of TBB's and Rayon's approach to task scheduling:
https://www.threadingbuildingblocks.org/docs/help/reference/task_scheduler/scheduling_algorithm.html

TBB offers two ways to spawn a task:

• task.execute() - similar to rayon::spawn() (push into the local LIFO queue)
• task.enqueue() - similar to rayon::spawn_fifo() (push into the global FIFO queue)

There's also a notion of task arenas, which I suppose is similar to Rayon's scopes.

Another interesting difference is how task stealing works. TBB first attempts to steal from the global FIFO queue and then steals from other workers' LIFO queues. Rayon uses the reverse order. I have a feeling that in this case TBB's strategy might be slightly better.

Docs are ready for review!

r=me with nits fixed

bors r=nikomatsakis

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