This prevents serialization and unserialization of a class and its
children in a way that does not depend on the zend_class_serialize_deny
and zend_class_unserialize_deny handlers that will be going away
in PHP 9 together with the Serializable interface.
In stubs, `@not-serializable` can be used to set this flag.
This patch only uses the new flag for a handful of Zend classes,
converting the remainder is left for later.
Closes GH-7249.
Fixes bug #81111.
We also change `Generator::throw()` to expect a `Throwable` in the
first place, and we now throw a TypeError instead of returning `false`
from `Exception::getTraceAsString()`.
