[NFR] Extend support for statement with other types

[gtux]

Hi, the For statement is very usefull, it will be good if you can extend the support for others than array or strings, for example: ArrayAccess or Traversable. In Phalcon there is many objects that I wanted to iterate in Zephir, it would be simple with for statement but it crash with "The argument is not initialized or iterable" message, so I have to change the for statement with while loop in order to get it work.