Began implementing composition-based traits

As described in <https://savannah.gnu.org/task/index.php#comment3>.

The benefit of this approach over definition object merging is primarily
simplicitly---we're re-using much of the existing system. We may provide
more tight integration eventually for performance reasons (this is a
proof-of-concept), but this is an interesting start.

This also allows us to study and reason about traits by building off of
existing knowledge of composition; the documentation will make mention of
this to explain design considerations and issues of tight coupling
introduced by mixing in of traits.