[RFC] Only unserialize Phar metadata when getMetadata() is called

In other words, don't automatically unserialize when the magic
phar:// stream wrappers are used.
RFC: https://wiki.php.net/rfc/phar_stop_autoloading_metadata

Also, change the signature from `getMetadata()`
to `getMetadata(array $unserialize_options = [])`.
Start throwing earlier if setMetadata() is called and serialization threw.

See https://externals.io/message/110856 and
https://bugs.php.net/bug.php?id=76774

This was refactored to add a phar_metadata_tracker for the following reasons:
- The way to properly copy a zval was previously implicit and undocumented
  (e.g. is it a pointer to a raw string or an actual value)
- Avoid unnecessary serialization and unserialization in the most common case
- If a metadata value is serialized once while saving a new/modified phar file,
  this allows reusing the same serialized string.
- Have as few ways to copy/clone/lazily parse metadata (etc.) as possible,
  so that code changes can be limited to only a few places in the future.
- Performance is hopefully not a concern - copying a string should be faster
  than unserializing a value, and metadata should be rare in most cases.

Remove unnecessary skip in a test(Compression's unused)

Add additional assertions about usage of persistent phars

Improve robustness of `Phar*->setMetadata()`

- Add sanity checks for edge cases freeing metadata, when destructors
  or serializers modify the phar recursively.
- Typical use cases of php have phar.readonly=1 and would not be affected.

Closes GH-5855

UPGRADING

@@ -425,6 +425,11 @@ PHP 8.0 UPGRADE NOTES
 - PDO_ODBC:
   . The php.ini directive pdo_odbc.db2_instance_name has been removed
 
+- Phar:
+  . Metadata associated with a phar will no longer be automatically unserialized,
+    to fix potential security vulnerabilities due to object instantiation, autoloading, etc.
+    RFC: https://wiki.php.net/rfc/phar_stop_autoloading_metadata
+
 - Reflection:
   . The method signatures