Use zval storage for php://memory stream

Description

The following code increases the peak memory usage of PHP twice:

$content = str_repeat('hello', 50000);
$h = fopen('php://memory', 'r+');
fwrite($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 (see https://github.com/Nyholm/psr7/pull/241).

Could it be possible to improve php://memory (and possibly php://temp) to make it use a zval storage to benefit from copy-on-write?

Empieza rastreando la implementación del stream php://memory y compara su comportamiento de almacenamiento con el wrapper personalizado respaldado por zval descrito en el issue. Determina cómo debería gestionarse php://temp y cómo afecta al diseño el compromiso de tamaño de aproximadamente 200K. Se considera terminado cuando el stream se beneficia de copy-on-write sin empeorar el rendimiento de las cadenas más pequeñas.