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.

Userland/Libraries/LibJS/Heap/Allocator.h

@@ -63,7 +63,7 @@ public:
 private:
     const size_t m_cell_size;
 
-    typedef IntrusiveList<HeapBlock, &HeapBlock::m_list_node> BlockList;
+    typedef IntrusiveList<HeapBlock, RawPtr<HeapBlock>, &HeapBlock::m_list_node> BlockList;
     BlockList m_full_blocks;
     BlockList m_usable_blocks;
 };

Userland/Libraries/LibJS/Heap/HeapBlock.h

@@ -86,7 +86,7 @@ public:
         return cell_from_possible_pointer((FlatPtr)cell);
     }
 
-    IntrusiveListNode m_list_node;
+    IntrusiveListNode<HeapBlock> m_list_node;
 
 private:
     HeapBlock(Heap&, size_t cell_size);