WIP fluent, template-based builders for message test vectors

[raulk]

Doesn't compile yet.

TODO

• [ ] Finish migrating all message-class test vectors to new fluent builders.
• [ ] Rich assertions.
• [ ] Independent go scripts.
• [ ] Use State Surgeon to perform state operations (i.e. move state-related Builder methods there).
• [ ] Move this out of tvx.