Kernel: Use intrusive RegionTree solution for kernel regions as well

This patch ports MemoryManager to RegionTree as well. The biggest
difference between this and the userspace code is that kernel regions
are owned by extant OwnPtr<Region> objects spread around the kernel,
while userspace regions are owned by the AddressSpace itself.

For kernelspace, there are a couple of situations where we need to make
large VM reservations that never get backed by regular VMObjects
(for example the kernel image reservation, or the big kmalloc range.)
Since we can't make a VM reservation without a Region object anymore,
this patch adds a way to create unbacked Region objects that can be
used for this exact purpose. They have no internal VMObject.)

Kernel/Forward.h

@@ -80,7 +80,6 @@ class Region;
 class SharedInodeVMObject;
 class VMObject;
 class VirtualRange;
-class VirtualRangeAllocator;
 }
 
 class Spinlock;