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.

sched/core.c

@@ -6,7 +6,6 @@
 #include <kernel/sched.h>
 #include <kernel/task.h>
 #include <kernel/thread.h>
-#include <kernel/vm-region.h>
 
 extern kern_status_t setup_kernel_task(void);
 extern kern_status_t setup_idle_task(void);