Follow-on from #26 . As the ASG type is the final target of the Covenant ASGBuilder, as well as the thing we will both serialize and deserialize, we should make it a lot more pleasant to work with than it currently is. We already get quite a lot of API potential from algebraic-graphs, but ultimately we should consider what our use cases are and how to make them as painless as possible.

#18 is related to this issue as well, but can be worked on independently of it.