[php-src] Issue #11106: Use zval storage for php://memory stream

Issue: https://github.com/php/php-src/issues/11106 Author: nicolas-grekas ### Description The following code increases the peak memory usage of PHP: ```php $content = str_repeat('hello', 50000); $h = fopen('php://temp', 'r+'); fwrite($h, $content); ``` The reason is that PHP cannot currently use copy-on-write when writing to the stream. This is something that can be worked around by using a custom stream wrapper that keeps data in a zval, as done in e.g. https://github.com/Nyholm/psr7/pull/230 Using this proved useful for strings longer than 200K. For smaller strings, using a custom stream wrapper has a higher cost than the memcopy. Could it be possible to improve php://memory to make it us a zval storage to benefit from copy-on-write?