Getting the function name for logging a warning/error message using
format_function_name() is expensive and shows up with quite a few
percent in the profiler.

Luckily, we can completely get rid of that overhead by only getting the
function name in case we actually have to warn. To do that, check the
number of arguments ourselves instead of using args.requireAtLeast() and
then move the call to format_function_name() into the if-conditions of
those checks.