Use zend_string* instead of char* to improve performance

[zhaoyanliang2]

In php7, a lot of char* is replaced by zend_string*, this helps reduce memory allocation and hash calculations. This should also apply to zephir that is still heavily using char*. I have tried this, but it currently only works on php7.2 and php7.3. This is enough for me, I don't think I have time to finish the rest. If other people are interested, you can refer to my changes. zhaoyanliang2/zephir

[sergeyklay]

@dreamsxin What do you think about this? Can we start moving in this direction?

[dreamsxin]

@sergeyklay Using zend_string is very convenient when using Zend API.