Problem:
- The implementation of `find` is coupled to the implementation of
`DoublyLinkedList`.
- `append` and `prepend` are implemented multiple times so that
r-value references can be moved from into the new node. This is
probably not called very often because a pr-value or x-value needs
to be used here.
Solution:
- Decouple the implementation of `find` from the class by using a
generic `find` algorithm.
- Make `append` and `prepend` be function templates so that they can
have binding references which can be forwarded.
