add __serialize and __unserialize

Author: grachevko

src/Enum.php

@@ -229,6 +229,16 @@ final public function unserialize($serialized): void
         $this->id = (int) $serialized;
     }
 
+    final public function __serialize(): array
+    {
+        return ['id' => $this->toId()];
+    }
+
+    final public function __unserialize(array $data): void
+    {
+        $this->id = (int) $data['id'];
+    }
+
     /**
      * @throws InvalidArgumentException
      */