Function groupBy

groupBy<K, V>(array: Iterable<V>, grouper: (item: V) => K): Map<K, V[]>
Groups data by a function grouper, returning data as a map with string keys and array values. Multiple values can be assigned to the same group.

grouper must yield a string designated group for a given item.
Type Parameters
- K
  Type of key to group by. Typically string.
- V
  Type of values
Parameters
- array: Iterable<V>
  Array to group
- grouper: (item: V) => K
  Function that returns a key for a given item
Returns Map<K, V[]>
Map
Example
```
import { Arrays } from 'https://unpkg.com/ixfx/dist/data.js';

const data = [
 { age: 39, city: `London` },
 { age: 14, city: `Copenhagen` },
 { age: 23, city: `Stockholm` },
 { age: 56, city: `London` }
];

// Whatever the function returns will be the designated group
// for an item
const map = Arrays.groupBy(data, item => item.city);
```
This yields a Map with keys London, Stockholm and Copenhagen, and the corresponding values.
```
London: [{ age: 39, city: `London` }, { age: 56, city: `London` }]
Stockhom: [{ age: 23, city: `Stockholm` }]
Copenhagen: [{ age: 14, city: `Copenhagen` }]
```