Associate block with metadata and fix transaction encoding

[shenkeyao]

• Updates the BlockPayload trait.
  • Add a Metadata type and a constructor to build the payload and metadata.
  • Add transaction encoding and decoding functions.
• Updates the BlockHeader constructor to take metadata as an input.
• Implements the new BlockPayload functions for the VIDBlockPayload struct.
• Fixes transaction flattening to include the length of each transaction.

Closes #2024. Closes #2025.

[ggutoski]

Some quick comments. Perhaps I'll post more later when I have time. We'll definitely need feedback from @jbearer here.

[shenkeyao]

All the task implementations (for consensus, transactions, etc.) are still using VIDBlockPayload which used to be the only payload struct. I should have replaced all of them in the production code with the app-specified payload type.

Will update this. Thanks for the comments! @jbearer @ggutoski

[jbearer]

I think the overall shape of things is right now. All of my comments are just details

[jbearer]

I think it looks good, except see this thread about possibly removing the commit() method of BlockPayload and having HotShot keep track of the commit on its own. WDYT about that @shenkeyao @ggutoski

[shenkeyao]

I think it looks good, except see this thread about possibly removing the commit() method of BlockPayload and having HotShot keep track of the commit on its own. WDYT about that @shenkeyao @ggutoski

@jbearer As mentioned in that thread, I think it makes sense to remove the Commitable bound. I've made an issue and will do that after merging this PR.