Move e-mail element around in registration form

Apparently it's very easy. I put this in registration_install() and that works:

function registration_install() {
	// Set up standard e-mail field
	$field = array(
		'field_name' => 'registration_email',
		'type' => 'email',
		'translatable' => FALSE,
		'locked' => TRUE,
	);
	$instance = array(
		'field_name' => $field['field_name'],
		'entity_type' => 'registration',
		'bundle' => 'registration',
		'label' => 'E-mail address',
		'widget' => array(
			'type' => 'email_textfield',
			'weight' => 0,
		),
	);

	// Create the field and instance
	try {
		field_create_field($field);
		field_create_instance($instance);
	}
	catch (Exception $e) {
		drupal_set_message(t('There was a problem creating field %label: @message.', array('%label' => $instance['label'], '@message' => $e->getMessage())));
	}
}

In the registration module, you'll then have to reference the entity field registration_email instead of the programatically added e-mail field. That shouldn't be too hard either.