Do not allow to override superglobals in views

  * Do not allow to override superglobals in views;
  * Use `zend_hash_merge_ex()` instead of manual iteration over the array and `phalcon_set_symbol()`
  * Get rid of the memory frame in `Phalcon\Mvc\View\Engine\Php::render()`
  * Make `Phalcon\Mvc\View\Engine::render()` return boolean
  * Optimize `Phalcon\Mvc\View\Engine\Volt::length()`
  * Optimize `Phalcon\Mvc\View\Engine\Volt::isIncluded()`