- worked it out so that classes declare a nested class "event",

- worked it out so that classes declare a nested class "event",
with methods representing events.  This is self-documenting via sphinx.
- implemented new model for pool, classmanager.   Most events are
one or two args, so going back to allowing any kind of *arg, **kw
signature for events - this is simpler and improves performance,
though we don't get the "we can add new kw's anytime".  perhaps
there's some other way to approach that.