vm: replace vm-region with address-space

address-space is a non-recursive data structure, which contains a flat list of vm_areas representing
mapped vm-objects.

userspace programs can no longer create sub-address-spaces. instead, they can reserve portions of
the address space, and use that reserved space to create mappings.

syscall/vm-object.c

@@ -3,7 +3,6 @@
 #include <kernel/sched.h>
 #include <kernel/syscall.h>
 #include <kernel/vm-object.h>
-#include <kernel/vm-region.h>
 
 kern_status_t sys_vm_object_create(
 	const char *name,