I'd like to see our serialization format start with a magic number and version. I'm thinking 8 bytes like: Aryn major16 minor16.

Overall, this seems like a sane way to use MessagePack, but I don't know enough about MessagePack to know if it'll satisfy our eventual needs.

I've now added magic bytes, a version header, and zero padding.

I still think we need to raise something other than ValueError in most cases. Generally, I think of ValueError as more of a programming error. I see agreement here: https://stackoverflow.com/questions/79590536/what-python-exception-should-i-raise-when-an-input-file-has-the-wrong-size

Done.