Use phalcon_is_iterable() macro instead of phalcon_is_iterable_ex() function

The macro was updated to restore the memory frame and the function was modified not to touch memory frames. This allows to use phalcon_is_iterable_ex() function in the kernel code where memory frames are avoided to boost performance.

Now that phalcon_is_iterable_ex() became smaller, it has more chances to be inlined when used in the kernel code.