[php-src] PR #23189: Optimize var_export

Pull Request: https://github.com/php/php-src/pull/23189 Author: kamil-tekiela The first commit replaces sprintf with memset. The second commit replaces php_addcslashes and php_str_to_str with a custom function based on the smart_str_append_escaped function from zend_smart_str.c. At one of the 3 call sites, the php_str_to_str was absent, and I didn't know what to do, which is why I added the flag. This makes the most sense to me, but I could also make it separate from the new function. I don't know whether buffer_append_spaces should have inline or not. I assume it shouldn't. In my simple round of testing, this proved to make var_export 3x faster.