Organize linked list files

- Moved linked list sort and test files into a LinkedList folder.
- Extract the linked list node class into a separate file.
- Imported relevant file in the test.

No new logic was added, this is purely a refactor commit.
Implement missing methods in NPLinkedList

Implemented a bunch of methods that were declared but not yet
implemented in NPLinkedList.

Also fixed a few little bugs in some of the methods.
Add 129 tests for NPLinkedList

Added 129 new tests to the NPLinkedList spec.  All tests pass.

The vast majority of public methods are tested.  There's still a few
that are left to do.