Improve "inner" constructor for DerivationTree

[monsterkrampe]

The inner constructor of the DerivationTree accepts PreDerivationTrees as children instead of directly expecting DerivationTrees. The latter seems more natural to me. In #28 the induction principle is already reworked and once the inner constructor is changed, the induction principle (recOn') can likely be further improved.

[monsterkrampe]

We started reworking this library to be closer to the lecture material so I think that this issue is no longer relevant.