Add practice problem to implement doubly linked list from scratch

[phrdang]

New Topic Request

Add a new section to Ch. 16 called "Do it Yourself: DoublyLinkedList" after the DIY SinglyLinkedList section (16.3) which includes an explanation of how to implement a DoublyLinkedList. (Of course, do not actually give the code, just an explanation of the algorithm or perhaps pseudocode.)

Why it's needed

It is mentioned in Ch. 16 that the built-in LinkedList is doubly linked but students never get the opportunity to actually implement this (only the singly linked and circular).

[phrdang]

Whoever implements this might as well do it on #33 .