Sum transaction miner fees in the block verifier

[teor2345]

Motivation

To validate the miner's spends in the coinbase transaction, we need to calculate the total miner fees for the block.

This is part of ticket #1162. It blocks PR #3067.

Specifications

This PR calculates the "the transaction fees paid by transactions in this block" part of this rule:

The total value in zatoshi of transparent outputs from a coinbase transaction, minus vbalanceSapling, minus vbalanceOrchard, MUST NOT be greater than the value in zatoshi of miner subsidy plus the transaction fees paid by transactions in this block.

https://zips.z.cash/protocol/protocol.pdf#txnencodingandconsensus

Solution

• Sum the miner fees for each block into a total amount
• Return an error if the calculation overflows
• Skip the coinbase transaction, because it doesn't contribute to miner fees
• Make sure the transaction verifier response is the correct type

Unrelated changes:

• Improve docs in the block verifier
• Move a slow check after all the quick checks
• Add some consensus rule TODOs

Review

@oxarbitrage wanted this for PR #3067.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Tests will be part of PR #3067.

Follow Up Work

• Validate coinbase miner subsidy and miner fees #1162 in PR Validate miner transaction fees #3067

[oxarbitrage]

Looks great, very appreciated.