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/Heap/SlabAllocator.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/Types.h>
+
+class JsonObjectSerializer;
+
+void* slab_alloc(size_t slab_size);
+void slab_dealloc(void*, size_t slab_size);
+void slab_alloc_init();
+void slab_alloc_stats(Function<void(size_t slab_size, size_t allocated, size_t free)>);
+
+#define MAKE_SLAB_ALLOCATED(type)                                        \
+public:                                                                  \
+    void* operator new(size_t) { return slab_alloc(sizeof(type)); }      \
+    void operator delete(void* ptr) { slab_dealloc(ptr, sizeof(type)); } \
+                                                                         \
+private: