dlist: Use a DoubleEndedIterator for .mut_iter() and .mut_rev_iter()

Unify the mutable iterators too. Switch the ListInsertion trait to use
method .insert_next() and .peek_next() for list mutation. .insert_next()
inserts an element into the list that will not appear in iteration, of
course; so the length of the iteration can not change during iteration.