Document and support nested composites

Document and support nested composites

Composites can behave in a "nested" fashion by defining the
class in that way.   To make the constructor more convenient,
a callable can be passed to :func:`.composite` instead of the
class itself.  This works now, so add a test to ensure this
pattern remains available.

Change-Id: Ia009f274fca7269f41d6d824e0f70b6fb0ada081