Avoid truncation on null bytes in class and function names

[iliaal]

Follow-up to phpGH-22681. _class_string() and _function_string() still format the class name and the function name with %s, so Reflection*::__toString() cuts both at the first NUL byte. Anonymous class names embed one, directly and also inside the generated name of a closure declared in such a class.

$obj = new class {
    public function make(): Closure { return function () {}; }
};
echo new ReflectionFunction($obj->make());
// Closure [ <user> public method {closure:class@anonymous ] {

master already appends the class name as a zend_string, so only the function name changes once this is merged up.

[iliaal]

Promoted upstream as php#22971.