AK+Kernel: Make IntrusiveList capable of holding non-raw pointers

This should allow creating intrusive lists that have smart pointers,
while remaining free (compared to the impl before this commit) when
holding raw pointers :^)
As a sidenote, this also adds a `RawPtr<T>` type, which is just
equivalent to `T*`.
Note that this does not actually use such functionality, but is only
expected to pave the way for #6369, to replace NonnullRefPtrVector<T>
with intrusive lists.

As it is with zero-cost things, this makes the interface a bit less nice
by requiring the type name of what an `IntrusiveListNode` holds (and
optionally its container, if not RawPtr), and also requiring the type of
the container (normally `RawPtr`) on the `IntrusiveList` instance.

Kernel/Scheduler.cpp

@@ -73,7 +73,7 @@ Atomic<bool> g_finalizer_has_work { false };
 READONLY_AFTER_INIT static Process* s_colonel_process;
 
 struct ThreadReadyQueue {
-    IntrusiveList<Thread, &Thread::m_ready_queue_node> thread_list;
+    IntrusiveList<Thread, RawPtr<Thread>, &Thread::m_ready_queue_node> thread_list;
 };
 static SpinLock<u8> g_ready_queues_lock;
 static u32 g_ready_queues_mask;