Provide a way to access a collection's elements sequentially without exposing its internal structure — a custom Collection should be usable in a for...of loop just like a built-in array.
Implement `Collection[Symbol.iterator]()`, returning an iterator object whose next() yields { value, done } pairs over the collection's items.
solve(items) adds every item to a Collection, iterates it with for...of, and returns each value doubled.
Sample tests