Ensure sequencer commitments are stable under serialization

[SuperFluffy]

The issue raised in #312 is more general than just tests: how can commitments be constructed and verified not only by the present library, but also by third parties and using different languages? The solution is to either define a stable/deterministic wire format, or by using a spec that promises a stable and deterministic set of bytes like https://borsh.io/

[SuperFluffy]

This issue has been partially circumvented by changing the payload that is being signed to a google.protobuf.Any in #1044. While this does not address the sequencer tests checking for protocol breakage to give false positives, it does address the issue of non-rust code (or rust code with a different prost-version) not encoding a byte-perfect wire payload.