Dispatch work to a thread pool

[dtolnay]

https://docs.rs/threadpool

[dtolnay]

@karan1276 mentioned being interested in working on this in https://github.com/brson/rust-cookbook/issues/24#issuecomment-285923636.

Possible problem statement: "Sort an array of M numbers using N threads"

[karan1276]

Demo i wrote for that can be found here: https://github.com/karan1276/rust-demo

[dtolnay]

Nice! Are you still working on turning this into a PR or would you need someone else to take over?

[leoyvens]

I think the example should use the Rayon thread pool, it's a more popular implementation.

[budziq]

I'd like to try this one.

It would be drawing a fractal (julia set) to an image with threadpool.