Adjust appends

[oxe-i]

1. Add an append to kindergarten-garden, since it is the first exercise where the student must construct a value using a proof.
2. Adjust high-scores append to make it clear do-notation is not necessary to "activate" destructive updates on arrays.
3. Adjust collatz-conjecture append to add a third possible approach which is often used to ensure termination: add an extra parameter for the max number of recursive calls.