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?

まず php://memory ストリームの実装を追跡し、そのストレージ動作を issue で説明されている zval ベースのカスタムラッパーと比較します。php://temp をどのように扱うべきか、また約 200K のサイズ上のトレードオフが設計にどのような影響を与えるかを判断します。ストリームが copy-on-write の恩恵を受けつつ、より小さい文字列のパフォーマンスを低下させないことが完了の条件です。