[kernel] Move kernel stacks out of the heap

We were previously allocating kernel stacks as large objects on the
heap. Now keep track of areas of the kernel stack area that are in use,
and allocate them from there. Also required actually implementing
vm_space::commit(). This still needs more work.

src/kernel/objects/thread.h

@@ -23,7 +23,6 @@ struct TCB
 	// TODO: move state into TCB?
 
 	uintptr_t kernel_stack;
-	size_t kernel_stack_size;
 
 	uint32_t time_left;
 	uint64_t last_ran;