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/Makefile

@@ -3,6 +3,7 @@ include ../Makefile.common
 KERNEL_OBJS = \
        init.o \
        Heap/kmalloc.o \
+       Heap/SlabAllocator.o \
        StdLib.o \
        Lock.o \
        Arch/i386/CPU.o \