Update method doc.
Introduce #to_linked_list for future conversion function.
Change conversion function name.
Update Node() doc.
Introduce List() conversion function.
Expose #first and #last as public api instead of #head and #tail.

For consistency methods #first and #last now return data stored instead
of node. Readers for #head and #tail are removed. Access to head and
tail is now only possible within the class as instance variables.