Minor stylistic to fixContent.
Adding missing case in deletion function.
Membership of Functor, Foldable, and Traversable.

Since we already have Data.Foldable.toList, toList is no longer
needed.

Note that AVLTree isn't actually a Functor in the categorical sense,
since we require the morphisms we fmap to be order-preserving if we
are to keep the usual BST semantics. Nevertheless, such an instance
is useful, so it is added.