Redesign Transaction V5 serialization, impl trusted vector security, nullifier utility functions

[oxarbitrage]

Motivation

It's easy to accidentally serialize V5 transaction fields in the wrong order. So we should go back to the design stage for V5 transaction serialization.

Solution

Design

• Update the v5 transaction RFC with an explicit design for serialization
  • Only have serialize and deserialize impls on types that are serialized as continuous bytes in V4 and V5 transactions
  • To avoid accidental serialization of Outputs in the V4 format in V5 transactions, add a wrapper type, and move the serialization impls to that type
  • List the types that have serialization impls, noting new impls
  • Explain how to serialize types that don't have serialization impls, by splitting those types up into their parts

Code

• Implement the design for sapling spends and outputs, to make sure it works
• Implement transaction nullifier utility functions
• Fix a bug in the overwinter flag in transaction v5 deserialization

Security

• Implement trusted vector preallocation for spends and outputs

The code in this pull request has:

• Documentation Comments
• Property Tests

Review

Related Issues

Follow Up Work

Implement V5 shielded data serialization - #1829

[teor2345]

I think we need to go back to the design stage with transaction v5 serialisation, because the spends and outputs are split into multiple arrays during serialisation.

[teor2345]

Nice catch here by the way 👍

I am happy with the current PR, waiting on @dconnolly to review

[teor2345]

The anchor field is only present when nSpendsSapling > 0:

Suggested change

	shared_anchor: AnchorV::Shared,
	// The following fields are in a different order to the serialized data, see:
	// https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus
	first: Either<Spend<AnchorV>, Output>,
	// The following fields are in a different order to the serialized data, see:
	// https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus
	first: Either<(Spend<AnchorV>, AnchorV::Shared), Output>,

[teor2345]

We've got 2 open PRs that depend on this PR, and we want to open 2 more, so we're just going to merge it.