Added Iterator trait to DoubleModList, small changes regarding hash function

Current changes based on the observation that each ListNode has a Key, a Value and a Next property. The iterator trait just provides these three methods, which enables code reduce without any additional reads.

To overcome covariance issues, a somewhat clumsy type definition was necassary. It is possible to include the trait into AdjustableList, however, I omitted these changes for now, since I did not want to change the code of ChunkedList. 

PS: Is there already a naming convention for Mods vs non-Mods?