@bill-myers mentioned about removing all iterator-based methods in favor of ```iter().{len, all, any, find, is_empty}```. What do you think?

As for my experience having ```iter()``` in list type is not possible using ```@```, as its matching pattern is now forbidden - ```Cons(ref head, @ref tail)``` - thus why I added ```BoxedListOps``` in the first place. Do you have a suggestion for implementing ```iter()```which doesn't require using ```@ref```?