[kutil] Make enum bitfields usable in other scopes

Changing the SFINAE/enable_if strategy from a type to a constexpr
function means that it can be defined in other scopes than the functions
themselves, because of function overloading. This lets us put everything
into the kutil::bitfields namespace, and make bitfields out of enums in
other namespaces. Also took the chance to clean up the implementation a
bit.

src/kernel/vm_space.h

@@ -132,4 +132,4 @@ private:
 	kutil::spinlock m_lock;
 };
 
-IS_BITFIELD(vm_space::fault_type);
+is_bitfield(vm_space::fault_type);