[NFR] Use native base64_encode/decode() and md5()

This pull requests replaces calls like `phalcon_call_func_p1(serialized, "base64_encode", data);` with a call to the native `php_base64_encode()`/`php_base64_decode()`. The advantage is that there is much less overhead to call a native function than to call a function via Zend API.

Same for `md5()`: it is replaces with calls to native `PHP_MD5Init/Update/Final()` functions.