#21 Circular lists

[somekindofwallflower]

Description
Given a linked list, return true if the list
is circular, false if it is not.

Examples
const l = new List();
const a = new Node('a');
const b = new Node('b');
const c = new Node('c');
l.head = a;
a.next = b;
b.next = c;
c.next = b;
circular(l) // true