Nested ACSet Serialization

[olynch]

There should be a native serialization of nested ACSets that is not Catlab-compatible because it is explicitly nested. Of course, at some point, we may want to support nested ACSets in Catlab.

Then we should have conversion to and from flat acsets. Flat acsets should have Catlab-compatible serialization.

[sjbreiner]

I'm going to start working on this, at least to get it to the point where I can save and retrieve work from a file.

[olynch]

Thanks @sjbreiner!