Objectives
 - Create a linked list
    - append(…)
 - Traverse a linked list
 - Free a linked list


Collections
-------------------
array -- all elements same type.
    Once array is created the size is fixed.
    You cannot add, remove, or reorder elements.
    You can access any element immediately.
struct -- fields (elements) may have different type
    Always same number per struct object
    You cannot add or remove fields.  Reordering fields would be pointless.
    You can access any field immediately.
linked lists
    All elements same type.
    Size can vary any time.
    You can add/remove/reorder elements at any time (without recreating).
    You must start at the head and access additional elements sequentially.