B.xs: move all B::*OP methods to B::OP::next

B.xs: move all B::*OP methods to B::OP::next

The previous commit moved all B::*OP methods capable of using direct field
offsets into next(). This commit moves the remaining B::*OP methods onto
it too (apart from oplist(), which returns a list rather than a single
item).

This simplifies the code, reduces the object size, and will also make it
easier to add an overlay facility, which will be coming soon.