PHP 7.4/NewMagicMethods: handle new __(un)serialize()

A new mechanism for custom object serialization has been added, which uses two new magic methods:

```php
// Returns array containing all the necessary state of the object.
public function __serialize(): array;

// Restores the object state from the given data array.
public function __unserialize(array $data): void;
```

The new serialization mechanism supersedes the Serializable interface, which will be deprecated in the future.

Refs:

• https://wiki.php.net/rfc/custom_object_serialization
• https://github.com/php/php-src/blob/3775d47eee38f3b34f800a0b23f840ec7a94e4c7/UPGRADING#L305-L315
• Implement new custom object serialization mechanism php/php-src#3761
• php/php-src@d373c11

Includes unit tests.

PHP 7.4/NonStaticMagicMethods: handle new __(un)serialize()

From the RFC:

Creating objects in __unserialize()

Some people have expressed a desire to make __unserialize() a static method which creates and returns the unserialized object (rather than first constructing the object and then calling __unserialize() to initialize it).

This would allow an even greater degree of control over the serialization mechanism, for example it would allow to return an already existing object from __unserialize().

However, allowing this would once again require immediately calling __unserialize() functions (interleaved with unserialization) to make the object available for backreferences, which would reintroduce some of the problems that Serializable suffers from. As such, this will not be supported.

Refs:

• https://wiki.php.net/rfc/custom_object_serialization
• https://github.com/php/php-src/blob/3775d47eee38f3b34f800a0b23f840ec7a94e4c7/UPGRADING#L305-L315
• Implement new custom object serialization mechanism php/php-src#3761
• php/php-src@d373c11

Includes unit tests.

Related to #808