Proposing a new 'cache_key' streams operation

Hi, PR is : https://github.com/php/php-src/pull/1711 This PR creates a mechanism to be used by opcode caches to determine whether a stream-wrapped URI is cacheable, and the key to use when caching it. The first usage for this operation is to opcode-cache the PHP code managed by PCS, which is essential when considering migrating part of a core extension from C to PHP. Please note that the PR *does not* include the changes to make opcache use this new mechanism. The reason is that the required changes are far from trivial, the opcache code is complex, and I prefer someone more familiar with it to implement the changes. Please comment. Regards François