Version 5.1.20

• Fix deadlocks when other apcu_* functions are used inside apcu_entry(). It should now be safe to use any functions inside the apcu_entry() callback.
• Fix division by zero exception in apc.php.
• Fix handling of references in PHP 8 if "default" serializer is used (which is not the default).
• Fix string reuse handling if "default" serializer is used (which is not the default).
• Check for failures when acquiring read locks to report problems earlier (write locks were already checked previously).
• Adjust tests for current PHP 8.1 development branch.
• Remove *_api.h headers. Use apc_cache.h instead of apc_cache_api.h etc.