Create types for unmined transactions and their IDs

[teor2345]

Motivation

Zebra's mempool needs to handle unmined v4 and v5 transactions.

These transactions have different unique identifiers.

Specifications

The transaction ID of a version 4 or earlier transaction is the SHA-256d hash of the transaction encoding in the
pre-v5 format described above.

The transaction ID of a version 5 transaction is as defined in [ZIP-244].

A v5 transaction also has a wtxid (used for example in the peer-to-peer protocol) as defined in [ZIP-239].

https://zips.z.cash/protocol/protocol.pdf#txnidentifiers
https://zips.z.cash/zip-0239
https://zips.z.cash/zip-0244

Solution

• Create an UnminedTxId enum, which holds a narrow or wide transaction ID, depending on version
  • Add accessor methods for the parts of an UnminedTxId
• Create an UnminedTx struct, which holds a transaction and its unmined ID

Review

This PR blocks merging other mempool PRs that handle transaction IDs.

It's based on PR #2618, so it might need a rebase on main when that PR merges.

Reviewer Checklist

• Code implements Specs
• Code makes sense

We'll test this code when we use these types in zebra-network and in zebrad's mempool.

Follow Up Work

Use these types in internal protocol messages in zebra-network.

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This change is 

[dconnolly]

These TxIds still apply post-mining though, right? They are not exclusively for un-mined transactions

[teor2345]

For v1-v4 transactions, the transaction::Hash is a unique identifier pre-mining (mempool and client) and post-mining (blockchain).

For v5 transactions, the WtxId uniquely identifies a pre-mined transaction.
But post-mining, only the transaction::Hash in their WtxId.id is required for unique identification.

This is because v5 transactions have a non-malleable transaction::Hash, which only covers the effects of the transaction (transparent and shielded spends and outputs). The authorizing data (proofs, signatures, and scripts) can change without changing the ID.

I've tried to update the documentation to clarify.
What do you think?

Suggested change

	pub enum UnminedTxId {
	/// A narrow unmined transaction identifier.
	///
	/// Used to uniquely identify transaction versions 1-4.
	Narrow(Hash),
	/// A wide unmined transaction identifier.
	///
	/// Used to uniquely identify transaction version 5.
	Wide(WtxId),
	}
	pub enum UnminedTxId {
	/// A narrow unmined transaction identifier.
	///
	/// Used to uniquely identify unmined version 1-4 transactions.
	/// (After v1-4 transactions are mined, they can be uniquely identified using the same [`transaction::Hash`].)
	Narrow(Hash),
	/// A wide unmined transaction identifier.
	///
	/// Used to uniquely identify unmined version 5 transactions.
	/// (After v5 transactions are mined, they can be uniquely identified using only their `WtxId.id`.)
	///
	/// For more details, see [`WtxId`].
	Wide(WtxId),
	}

[teor2345]

Doc updates to clarify wide vs narrow transaction IDs.

[teor2345]

For v1-v4 transactions, the transaction::Hash is a unique identifier pre-mining (mempool and client) and post-mining (blockchain).

For v5 transactions, the WtxId uniquely identifies a pre-mined transaction.
But post-mining, only the transaction::Hash in their WtxId.id is required for unique identification.

This is because v5 transactions have a non-malleable transaction::Hash, which only covers the effects of the transaction (transparent and shielded spends and outputs). The authorizing data (proofs, signatures, and scripts) can change without changing the ID.

I've tried to update the documentation to clarify.
What do you think?

Suggested change

	pub enum UnminedTxId {
	/// A narrow unmined transaction identifier.
	///
	/// Used to uniquely identify transaction versions 1-4.
	Narrow(Hash),
	/// A wide unmined transaction identifier.
	///
	/// Used to uniquely identify transaction version 5.
	Wide(WtxId),
	}
	pub enum UnminedTxId {
	/// A narrow unmined transaction identifier.
	///
	/// Used to uniquely identify unmined version 1-4 transactions.
	/// (After v1-4 transactions are mined, they can be uniquely identified using the same [`transaction::Hash`].)
	Narrow(Hash),
	/// A wide unmined transaction identifier.
	///
	/// Used to uniquely identify unmined version 5 transactions.
	/// (After v5 transactions are mined, they can be uniquely identified using only their `WtxId.id`.)
	///
	/// For more details, see [`WtxId`].
	Wide(WtxId),
	}

[teor2345]

Some comment clarifications about v5 transactions