Proposal to add `groupFlatMap`

for every `groupMap`

```
def groupFlatMap[K, B](key: A => K)(f: A => IterableOnce[B]): immutable.Map[K, CC[B]]
```

is to

```
def groupMap[K, B](key: A => K)(f: A => B): immutable.Map[K, CC[B]]
```

as

```
def flatMap[B](f: A => IterableOnce[B]): CC[B]
```

is to

```
def map[B](f: A => B): CC[B]
```

Would groupFlatMapReduce also be useful?

…

After further thought, maybe its not obvious what would happen if IterableOnce is empty.