The idea was that the Python argument spec used normalised names (type, callable) for both adapters and converters, but the C code used more direct names, as I found it slightly more readable. However, it does add little value, now that I look at it again. (It also helps with new eyes on the code.)

I still want to normalise the Python argument spec in some kind of way, but it is not very important for me, I can leave it out. Currently register_adapter uses (type, caster), and register_converter uses (name, converter).