[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/apic.h

@@ -3,7 +3,7 @@
 /// Classes to control both local and I/O APICs.
 
 #include <stdint.h>
-#include "kutil/enum_bitfields.h"
+#include "enum_bitfields.h"
 
 enum class isr : uint8_t;