Kernel: Add basic fixed width allocator for userspace

We have to move process stacks to the general heap and maybe map
kernel to higher half.

kernel/CMakeLists.txt

@@ -25,6 +25,7 @@ set(KERNEL_SOURCES
 	kernel/Input/PS2Keymap.cpp
 	kernel/InterruptController.cpp
 	kernel/kernel.cpp
+	kernel/Memory/FixedWidthAllocator.cpp
 	kernel/Memory/Heap.cpp
 	kernel/Memory/kmalloc.cpp
 	kernel/Panic.cpp