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?

Start by tracing the php://memory stream implementation and compare its storage behavior with the custom zval-backed wrapper described in the issue. Determine how php://temp should be handled and how the roughly 200K size trade-off affects the design. Done means the stream benefits from copy-on-write without regressing smaller-string performance.