vm: add vm-region to manage userspace virtual memory address spaces

vm-region supports creating nested regions of virtual memory, each with their
own memory protection restrictions.

vm-objects can be mapped into a vm-region, making the underlying memory
accessible. all mappings are lazy: page tables are not updated until the
mapped memory is accessed.

vm/bootstrap.c

@@ -4,6 +4,7 @@
 #include <mango/printk.h>
 #include <mango/status.h>
 #include <mango/vm-object.h>
+#include <mango/vm-region.h>
 #include <mango/vm.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -41,6 +42,7 @@ kern_status_t vm_bootstrap(
 
 	kmalloc_init();
 	vm_object_type_init();
+	vm_region_type_init();
 	return KERN_OK;
 }