Worker pool

[mre]

What?

Implement a worker pool

Why?

Show how to use channels similar to the go implementation. Maybe there's a way to use custom derives to create a worker pool from a "pool" struct

How?

Start with the go implementation, port it to Rust and then make it more idiomatic.

[mre]

Also see:

• https://github.com/Jeffail/tunny
• https://github.com/kinghajj/kirk
• https://docs.rs/threadpool/1.7.1/threadpool/
• https://docs.rs/rayon/*/rayon/struct.ThreadPool.html