add statics for RRClass::IN() (RRClass.IN()) etc.
(and replace RRClass("IN") in tests with those)

__str__ for name, question, everything with to_text()

All constructors based on buffers need an optional position
argument (like question_python has now)

at question.to_wire(bytes) does not seem to work right (only return
value seems correct, while i'd like in-place addition if possible)