Sacrilegious's way to get forms in templates for D8

theme.theme

/**
 * Provides suggestions for different parts, so that blocks are properly loaded
 *
 * @see hook_theme_suggestions_alter()
 *
 * @param array $suggestions
 * @param array $variables
 * @param string $hook
 */

function hook_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
  switch ($hook) {
    case 'form':
      if (isset($variables['element']['#form_id'])) {
        $suggestions[] = $hook . '__' . $variables['element']['#form_id'];
      }
    break;
  }
}

/**
 * @implements template_preprocess_HOOK()
 *
 * @param array $variables
 */
function hook_preprocess_form(&$variables) {
  if (isset($variables['element'])) {
    foreach (\Drupal\Core\Render\Element::children($variables['element']) as $key) {
      $variables['form'][$key] = $variables['element'][$key];
      unset($variables['form'][$key]['#markup']);
      $variables['form'][$key]['#printed'] = FALSE;
    }
  }
}