Description
Return the 'middle' node of a linked list.
If the list has an even number of elements, return
the node at the end of the first half of the list.
Do not use a counter variable, do not retrieve
the size of the list, and only iterate
through the list one time.

Example
const l = new LinkedList();
l.insertLast('a')
l.insertLast('b')
l.insertLast('c')
midpoint(l); // returns { data: 'b' }