Add a method in solutions/18_iterators/iterators4.rs

[1vk3y]

Add a method that solves the problem: using recursion combined with match to implement factorial.

[mo8it]

This isn't Haskell :P
In most cases, iterative solutions are more effecient than recursive ones. I don't want to recommend recursion where it isn't needed.

[1vk3y]

Touché. :D