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.

include/kernel/task.h

@@ -24,7 +24,7 @@ struct task {
 	char t_name[TASK_NAME_MAX];
 
 	pmap_t t_pmap;
-	struct vm_region *t_address_space;
+	struct address_space *t_address_space;
 	spin_lock_t t_handles_lock;
 	struct handle_table *t_handles;
 	struct btree b_channels;