[PHP-DEV] [RFC] [Discussion] Add bundled user_cache extension

Hi internals,

This is a follow-up to the previous "OPcache Static Cache" discussion.

I have written a new RFC and implementation. It addresses the issues
raised in the previous thread, drops the parts that led to feature
creep, and moves from OPcache integration to a new bundled
`user_cache` extension.

(The `OPcache User Cache` entry in the commit history is a remnant of
a completely new implementation built from scratch.
After realizing partway through that it wasn't necessary to depend on
OPcache, I separated it.)

RFC: https://wiki.php.net/rfc/impl_user_cache
Implementation: https://github.com/php/php-src/pull/22730

The scope and API have changed a lot from the previous proposal, so I
am starting this as a new RFC with its own discussion period. Because
of that, I am not sure it can make the PHP 8.6 feature freeze. If the
release managers think 8.6 is not realistic, I will target 8.7.

As mentioned in the previous thread, I think library developers would
benefit from having this available sooner, so feedback is welcome.

Best regards
Go Kudo