Allow each helper to accept PHP Generators

Output from the following function:

```php
function getItems() {

   //Some code

   while( $stream->more() ) {
      yield [ 'id' => $stream->id(), 'text' => $stream->text() ];
   }
}

```

Can now be directly fed as follows:

```handlebars

{{#each items}}
   {{id}} - {{text}}
{{/each}}

```