Allow iterables in traverseAllChildren

This lets you use any kind of iterable as a container of react child elements. It also preserves keys when possible if the iterable is keyed.

 * Use an ES6 Map or Set to hold children.
 * Use an Immutable-js collection to hold children.
 * Use a function* which yields children.

Concretely, this removes the necessary conversion to JS objects if you're mapping over Immutable-js collections to get children, i.e.:

```
<div>
  {myImmutable.map(val => <span>{val}</span>).toJS()
</div>
```

Now we can remove the `toJS()` and the intermediate allocation along with it.
No, because if the iterable is not returning entries then its returning any value, and traverseAllChildren will do the right thing with it. The value iteration below should mirror the Array loop above.