JSON serialization for diagram of ACSets

[aaguinal]

Per @jpfairbanks 's suggestion: it would be nice to be able to serialize a diagram like a span of acsets.

[epatters]

This would be nice. Since we can already serialize acsets, the remaining features are:

1. Serialize an acset transformation
2. Serialize a diagram in a category (in this case, the category of acsets)
3. Serialize a diagram morphism, enabling serialization of diagrams of diagrams (@kris-brown notes that this would be useful in certain projects)

Since an acset can itself be regarded as a diagram in Set, and acset transformations as diagram morphisms with identity map between indexing categories, it seems possible that everything could be unified through two features: serializing diagrams and their morphisms. I haven't thought through how practical that would be.