WIP - Validate shielded coinbase

[oxarbitrage]

TODO

• Replace Either with a custom type
• Check we've implemented all the ZIP-213 rules
• Add tests
• Merge

Motivation

Shielded coinbase are available after Heartwood NU. Consensus rules for this are described at ZIP-213

Solution

Implement shielded_coinbase() function to the subsidy::general that makes rules validation.

Related Issues

• ZIP-213: Validate Shielded Coinbase Outputs like other shielded outputs #608

• Draft RFC: Block subsidy #1129

[dconnolly]

We can answer this question without a shields() method, by destructuring the struct to get shielded_data, which is an Option(ShieldedData). If shielded_data == None, that means there are no Spend descriptions or Output descriptions. Some examples in https://github.com/ZcashFoundation/zebra/pull/1174/files

[oxarbitrage]

I don't think we should be destructuring inside subsidy. If we do it to get shielded_data we will have to do it for outputs() or joinsplits() for consistency. I might be missing something but i see the method as the best option here.

I did some cleaning to the method and renamed to shielded_data() in a4d2644

[teor2345]

Let's replace Either<...> with a named and typed enum.

We could do something like:

enum JoinSplit<'_> {
    JoinSplitOnBctv14(&'_ JoinSplitData<Bctv14Proof>),
    JoinSplitOnGroth16(&'_ JoinSplitData<Groth16Proof>),
}

You'll need lifetimes here, the compiler should give you hints if I've got it wrong.

[teor2345]

We decided to close this draft PR, and re-open it when we start working on ticket #608 again.