Implement deepEqual

Write deepEqual(a, b) returning whether two values are structurally equal. Values are primitives, arrays, or plain objects, nested to any depth, with no cycles.

• Primitives compare by value; NaN equals NaN, and 0 equals -0.
• Arrays are equal when they have the same length and pairwise-equal elements; an array never equals an object.
• Objects are equal when they have the same own keys and equal values for each key. A key present with the value undefined is still a key.

deepEqual({ a: [1, { b: null }] }, { a: [1, { b: null }] })  // true
deepEqual([1, 2], [2, 1])                                    // false
deepEqual({ a: 1 }, { a: 1, b: undefined })                  // false