drupal_render() should check if it's rendering a 'render element' and if so call drupal_render_children() (inline) instead

We are recommending not to call drupal_render() in preprocess, now formally documented here: http://drupal.org/node/1920746

Per #1898432-20: node.module - Convert PHPTemplate templates to Twig → , the same should be true of drupal_render_children().

drupal_render_children should be implicit when something is printed using twig, so that we don't have to do it in preprocess.