ComposableListSolution

- answers in ComposableListSolution
- start of unit testing for ComposableListSolution
- no unit test yet to compare against ComposableListExercises
- split up types and interfaces as needed to work in Java (no dynamic typing like Javascript to allow comparing different implementations without shared types)
- fixes a few small issues
- questions 24/25 left without answers, look like they need to be finished (no comments on them)