Double-Linked Skip List and Arbitrary Seeking Support

Hey Ric,

Attached is an implementation of the Skip List being double-linked for reverse traversal.  Secondarily there is support for arbitrary seeking.  If you want me to refactor this into a separate type so that the base implementation remains the same (except for keeping the arbitrary seeking support in both), I can do that.  Please let me know how the base implementation looks, and I'll start the refactoring afterwards.

This is per https://github.com/ryszard/goskiplist/issues/1.

Cheers,

Matt