[includes] Move enum_bitfields.h to base includes

Pull this widely-useful header out of kutil, so more things can use it.
Also replace its dependency on <type_traits> by defining our own custom
basic_types.h which contains a subset of the standard's types.

src/kernel/objects/vm_area.h

@@ -5,8 +5,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "enum_bitfields.h"
 #include "j6/signals.h"
-#include "kutil/enum_bitfields.h"
 #include "kutil/vector.h"
 
 #include "kernel_memory.h"