Exercises that teaches modules

In #2 the idea of an exercise that requires writing a module, with exports, public interface etc. came up. This is quite important for writing reusable Julia code.

Many of the exercises are too simple for requiring this. My initial idea is that an exercise like Binary Search Tree, (Simple) Linked List or List Ops would work well for it. With the v2 progression system, we could add one of them as a core exercise that unlocks the others and then combine them in a way that the user has to write and extend a DataStructures-module. This is an actual Julia [package](https://github.com/JuliaCollections/DataStructures.jl), so it would be a very practical example.