Adding a scoped function

[dns2utf8]

I wonder if it would be useful to have a scoped function similar to the scoped_threadpool crate.

Advantages could come from the design. Like:

• dedicate only a certain part of the pool to the scoped work
• add the scoped jobs in random order but join the whole pool at the end of the scoped function
• add the scoped jobs in random order but join only the scoped jobs

One could also state that the resources would be wasted since such a crate already exists.

[frewsxcv]

another example:

https://docs.rs/rayon/0.8.2/rayon/struct.ThreadPool.html#method.scope