### promisify

Converts an asynchronous function to return a promise.

Use currying to return a function returning a `Promise` that calls the original function.
Use the `...rest` operator to pass in all the parameters.

*In Node 8+, you can use [`util.promisify`](https://nodejs.org/api/util.html#util_util_promisify_original)*

```js
const promisify = func => (...args) =>
  new Promise((resolve, reject) =>
    func(...args, (err, result) => (err ? reject(err) : resolve(result)))
  );
```

```js
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2s
```