B.xs: add class names to method branch comments

A single function in B.xs handles the method calls for a whole bunch of
methods from a whole bunch of subclasses (e.g. B::OP, B::COP etc).

Extend the comment next to each switch case to include the full method
name: e.g. "B::COP::stash" rather than just "stash".

This makes things slightly less confusing.