[8.x] Adds PHP 8.1 support to serializable closures

[nunomaduro]

This pull request adds PHP 8.1 support to serializable closures by replacing opis/closure by laravel/serializable-closure.

• We decided to fork the opis package as the upcoming version 4.x is a complete rewrite on top of the FFI extension. As Laravel is a web framework, and FFI is not enabled by default in web requests (and some OS's), this fork allows us to keep using the 3.x series while adding support for new PHP versions - including PHP 8.1.
• Queued jobs, and cached routes, that were serialized with opis/closure will still work, as the package and the code still exists in Laravel 8.x. Yet, the package opis/closure is scheduled for removal in Laravel 9.x. By then, normally people no longer have queued jobs, and cached routes, using the old format. Of course, every new queued jobs, and cached routes, will be using laravel/serializable-closure for serialization.
• Small caveat: Queued jobs, and cached routes, that used opis/closure for their serialized string formats won't work with PHP 8.1. Therefore, people need to ensure that all queued jobs, and cached routes, are already using the new laravel/serializable-closure before migrating to PHP 8.1. One idea to speed up the process is having this pull request targeting 8.x branch, instead of php81-fixes.

Remaining tasks before merging this pull request:

• Review the code on laravel/serializable-closure.
• Review this pull request.
• Decide if we target 8.x branch, instead of php81-fixes, so people can start using these changes sooner.
• Ask Nuno to tag a stable version of laravel/serializable-closure.

[GrahamCampbell]

👍 assuming this has been tested on a real app. :)

[agustinprod]

We're seeing some exceptions thrown on our queues, because both of the libraries are trying to register the same stream protocol:

stream_wrapper_register(): Protocol closure:// is already defined.

opis/closure author says he gained the right to dont check if it's in use:
opis/closure#106

Maybe laravel/serializable-closure should register a different protocol name

[nunomaduro]

Please create a new issue on the serializable closure repository.