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/Process.h

@@ -120,7 +120,7 @@ protected:
     mode_t m_umask { 022 };
     VirtualAddress m_signal_trampoline;
     Atomic<u32> m_thread_count { 0 };
-    IntrusiveList<Thread, &Thread::m_process_thread_list_node> m_thread_list;
+    IntrusiveList<Thread, RawPtr<Thread>, &Thread::m_process_thread_list_node> m_thread_list;
     u8 m_termination_status { 0 };
     u8 m_termination_signal { 0 };
 };