The :class:`.MutableComposite` type did not allow for the

The :class:`.MutableComposite` type did not allow for the
:meth:`.MutableBase.coerce` method to be used, even though
the code seemed to indicate this intent, so this now works
and a brief example is added.  As a side-effect,
the mechanics of this event handler have been changed so that
new :class:`.MutableComposite` types no longer add per-type
global event handlers.  Also in 0.8.0b2.
[ticket:2624]