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/PhysicalPage.h

@@ -2,6 +2,7 @@
 
 #include <AK/NonnullRefPtr.h>
 #include <Kernel/Assertions.h>
+#include <Kernel/Heap/SlabAllocator.h>
 #include <Kernel/VM/PhysicalAddress.h>
 
 class PhysicalPage {
@@ -9,6 +10,7 @@ class PhysicalPage {
     friend class PageDirectory;
     friend class VMObject;
 
+    MAKE_SLAB_ALLOCATED(PhysicalPage)
 public:
     PhysicalAddress paddr() const { return m_paddr; }