Kernel: Add Memory::RegionTree to share code between AddressSpace and MM

RegionTree holds an IntrusiveRedBlackTree of Region objects and vends a
set of APIs for allocating memory ranges.

It's used by AddressSpace at the moment, and will be used by MM soon.

Kernel/CMakeLists.txt

@@ -170,6 +170,7 @@ set(KERNEL_SOURCES
     Memory/PhysicalZone.cpp
     Memory/PrivateInodeVMObject.cpp
     Memory/Region.cpp
+    Memory/RegionTree.cpp
     Memory/RingBuffer.cpp
     Memory/ScatterGatherList.cpp
     Memory/ScopedAddressSpaceSwitcher.cpp