- on_expire event, since we are starting to build off of events around

- on_expire event, since we are starting to build off of events around
full lifecycle
- composite will use events to do everything we want it to, i.e.
storing the composite in __dict__, invalidating it on change
of any of the columns.
- will reinstate mutability of composites via callable attached
to the composite - but userland code will still need to establish
change event listening on the composite itself, perhaps via
a "mutable" mixin like the scalars.py example, perhaps via
addition of descriptors to the mutable object.