[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/libraries/elf/include/elf/headers.h

@@ -1,6 +1,6 @@
 #pragma once
 #include <stdint.h>
-#include "kutil/enum_bitfields.h"
+#include "enum_bitfields.h"
 
 namespace elf {
 
@@ -95,4 +95,4 @@ struct section_header
 
 } // namespace elf
 
-IS_BITFIELD(elf::section_flags);
+is_bitfield(elf::section_flags);