Change RR.len() to RR.Len()

To be able to predict the buffer size required by `PackRR()`, `RR.len()` should be exposed. Unfortunately the function is used a lot internally, so quite of few LOC need to be changed. If you can think of a better solution that would be great, but this seems like the most maintainable going forward.

Additionally, a useful complimentary function (not included in this PR) would be Question.Pack() along with Question.Len(). Should I open a separate PR for that?