Improve experience for plugging in different proof formats

[lightclient]

I think it would be beneficial to improve the "pluggability" of proof formats other than the offset-multiproof. I've been trying to open up the design of Sheth to allow for this and it is getting closer. The things that would really mark this as complete would be:

• composer tool has a repository of proof formats it can generate for
• each format implements the State trait so it can be used as an account database
• is rigorously tested -- the rest of the test in Sheth are focused on correctness of the logic and not the correctness of the storage unit
• added to benches + Makefile