DoublyLinkedList: Implement find in terms of AK::find

Problem:
- The implementation of `find` is coupled to the implementation of
  `DoublyLinkedList`.
- `append` and `prepend` are implemented multiple times so that
  r-value references can be moved from into the new node. This is
  probably not called very often because a pr-value or x-value needs
  to be used here.

Solution:
- Decouple the implementation of `find` from the class by using a
  generic `find` algorithm.
- Make `append` and `prepend` be function templates so that they can
  have binding references which can be forwarded.

Kernel/Devices/Device.h

@@ -75,7 +75,7 @@ public:
         {
             ScopedSpinLock lock(m_requests_lock);
             was_empty = m_requests.is_empty();
-            m_requests.append(request);
+            m_requests.append(RefPtr<AsyncDeviceRequest>(request));
         }
         if (was_empty)
             request->do_start({});