Add a set of closure exercises before iterators exercise.

[divyaranjan1905]

I believe another set of exercises should be there before the 19th set on interators. This would keep it consistent with The Book and also provide a better treatment for closures which aren't exactly intuitive to beginning Rustaceans. Closures are also good practice for functional programming.

If this could be done, I'd be glad in creating a set of exercises to this end as a PR.

[mo8it]

Very good idea, but duplicate of #1746. Let's discuss it there :)