### mapKeys

Creates an object with keys generated by running the provided function for each key and the same values as the provided object.

Use `Object.keys(obj)` to iterate over the object's keys.
Use `Array.reduce()` to create a new object with the same values and mapped keys using `fn`.

```js
const mapKeys = (obj, fn) =>
  Object.keys(obj).reduce((acc,k) => {acc[fn(obj[k], k, obj)] = obj[k]; return acc;},{});
```

```js
mapKeys({ 'a': 1, 'b': 2 }, (val, key) => key + val); // { a1: 1, b2: 2 }
```