Provide iterator-based approach for collections::list

[brunoabinader]

As discussed in PR #12160, we need an iterator-based approach for collections::list helper functions.

This task includes:

• Add a public type for @List<T> (i.e. BoxedList<T>)
• Create a trait for handling BoxedList<T> operations (i.e. BoxedListOps<T>)
• Create a BoxedListIterator<T> and implement iter() from within BoxedListOps<T>
• Refactory current helper functions (i.e. any(), has()...) to an iterator-based approach
• Move helper functions to BoxedListOps<T> so they can become methods w/ 'self' (i.e. list.has(1u) instead of list::has(list, 1u))