Save / Load Proofs from json / local storage

[eddiekrystowski]

We should be able to save and load proofs both from browser local storage and from .json file like with graphs.

Extra consideration will need to be taken to efficiently store hypergraphs as they can be quite large and most of the data does not need to be stored. Especially with non-linear hypergraphs and circular hypergraphs, they cannot be saved as-is because we cannot store circular references.