vm: add vm-object to represent non-contiguous physical memory allocations

vm-object can be used to demand-allocate non-contiguous physical memory, and
will provide an api for userspace programs to do the same. unless a vm-object
is created in-place (i.e. to represent a specific area of physical memory),
its memory pages are only allocated when the object is mapped AND someone
attempts to access the memory.

vm/bootstrap.c

@@ -3,6 +3,7 @@
 #include <mango/memblock.h>
 #include <mango/printk.h>
 #include <mango/status.h>
+#include <mango/vm-object.h>
 #include <mango/vm.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -39,6 +40,7 @@ kern_status_t vm_bootstrap(
 	}
 
 	kmalloc_init();
+	vm_object_type_init();
 	return KERN_OK;
 }