Implement `frequency(arr)` that returns an object mapping each unique element of `arr` to the number of times it appears.
Example: `frequency(['a','b','a','c','b','a'])` → `{ a: 3, b: 2, c: 1 }`.
Hints
Use a plain object as a counter.
For each element, increment its count in the map (defaulting to 0 if missing).