map.keys

```coffeescript
map.keys(mapper (key string)=>Promise|string)(object Object) -> mappedKeys Promise|Object
```

Apply a mapper concurrently to an object's keys, returning an object with mapped keys.

```javascript
console.log(
map.keys(key => `#${key}`)({ a: 1, b: 2, c: 3 }),
) // { '#a': 1, '#b': 2, '#c': 3 }
```