libluna: Add LinkedList tags

libluna/include/luna/LinkedList.h

@@ -8,7 +8,7 @@ template <typename T> class LinkedListNode
 {
     using SelfType = LinkedListNode<T>;
 
-  private:
+  protected:
     SelfType* m_next_node;
     SelfType* m_last_node;
 
@@ -284,3 +284,29 @@ template <typename T> class LinkedList
 
     usize m_count = 0;
 };
+
+struct Tag : public LinkedListNode<Tag>
+{
+  public:
+    Tag(void* ptr) : m_ptr(ptr)
+    {
+    }
+
+    ~Tag()
+    {
+        detach_from_list();
+    }
+
+    void* ptr()
+    {
+        return m_ptr;
+    }
+
+    template <typename T> T& as()
+    {
+        return *static_cast<T*>(m_ptr);
+    }
+
+  private:
+    void* m_ptr;
+};