Use fixed genesis coinbase data in generated genesis blocks

[teor2345]

Motivation

Zebra serializes genesis blocks with arbitrary coinbase data, but our deserialization checks for fixed mainnet/testnet coinbase bytes.

This causes failures in some proptests.

This is unscheduled work due to a Zebra bug / spec bug.

Specifications

The current Zcash specification for the genesis coinbase height encoding is incorrect. (zcash/zips#540)

Solution

• Return an error when trying to serialize a genesis height with non-genesis coinbase data
• Use fixed genesis coinbase data in generated genesis blocks
• Document input, coinbase, and height encoding and decoding functions
• Delete some documentation that is incorrect after recent spec changes

Review

This fix blocks PR #2519, so @jvff can review it.

Reviewer Checklist

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

Follow Up Work

We should get the spec fixed, then implement whatever it says.

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This change is 

[jvff]

Thanks for this! 😄

Reviewed 3 of 3 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @teor2345)

[daira]

I don't think there is a spec bug here. The genesis block (at height 0 of any chain) is explicitly excluded from coinbase height checks in section 7.1.2 of the protocol spec:

and in section 7.12:

[teor2345]

I don't think there is a spec bug here. The genesis block (at height 0 of any chain) is explicitly excluded from coinbase height checks in section 7.1.2 of the protocol spec

I'm still not sure how implementations are meant to identify genesis blocks, and distinguish them from blocks at height 520,617,983. But I think it's ok if we don't want to specify a consistent algorithm here.

See my detailed comment:
zcash/zips#540 (comment)