* make the doubly linked list more efficient
(both memory and speed when accessing individual elements)
* add configuration options, e.g.:
- Reverse => new elements are inserted in front
- AppendExisting => storing values to existing keys will
append (or prepend in case of Reverse)
this key
* add more compatibility with Tie::IxHash
(i.e. push, pop, shift, unshift, splice, ...)
* add more tests
* add more documentation