Kernel: Add a simple slab allocator for small allocations

This is a freelist allocator with static size classes that works as a
complement to the generic kmalloc(). It's a lot faster than kmalloc()
since allocation just means popping from the freelist.

It's also significantly more compact when there are a lot of objects
smaller than the minimum kmalloc chunk size (32 bytes.)

This patch enables it for the Region and PhysicalPage classes.
In the PhysicalPage (8 bytes) case, it's a huge improvement since we
no longer waste 75% of the storage allocated.

There are also a number of ways this can be improved, so let's keep
working on it going forward.

Kernel/VM/Region.h

@@ -1,18 +1,20 @@
 #pragma once
 
-#include <AK/String.h>
 #include <AK/Bitmap.h>
 #include <AK/InlineLinkedList.h>
+#include <AK/String.h>
+#include <Kernel/Heap/SlabAllocator.h>
 #include <Kernel/VM/PageDirectory.h>
 #include <Kernel/VM/RangeAllocator.h>
 
 class Inode;
 class VMObject;
 
-class Region : public RefCounted<Region>
+class Region final : public RefCounted<Region>
     , public InlineLinkedListNode<Region> {
     friend class MemoryManager;
 
+    MAKE_SLAB_ALLOCATED(Region)
 public:
     enum Access {
         Read = 1,