add_set_handler, set_content.

Moved the description of the lookup to the methods that actually
do the lookup.  Also found that I'd misunderstood what __qualname__
was, so I needed to add another level of lookup: the full name,
which includes the __module__ path.

Are all these lookups worth it?  Should the MRO be walked for
each in turn instead of doing all four on a type and *then*
walking?  Open questions.