Implement transaction::check for Transaction Version 5 and Orchard

[teor2345]

Is your feature request related to a problem? Please describe.

After we parse Sapling in #1829 and Orchard in #1979, we need to implement and test the transaction checks for transaction version 5.

Describe the solution you'd like

These checks depend on some of the Orchard action functions in #2185, we're splitting them out into a separate PR:
https://github.com/ZcashFoundation/zebra/pull/2185/files#r638307833

has_inputs_and_outputs

• copy the orchard consensus rules into the function docs - https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus
• implement for transaction version 5:
  • add an orchard_actions method to Transaction, structured like sapling_nullifiers, but using Actions from orchard_shielded_data.actions()
  • count the number of actions in has_inputs_and_outputs
  • add the number of actions to the total number of inputs, and the total number of outputs (they can contain either or both)
• add Orchard tests (see details below)

shielded_balances_match

• document that the value_balance field in sapling::ShieldedData is only present when there is at least one spend or output
  • copy the consensus rule here?
• during V4 sapling deserialization, return an error if ShieldedData is None, and the value balance is non-zero
  • copy the consensus rule here?
• during V4 sapling serialization, write a zero value balance if there is no ShieldedData
• delete the shielded_balances_match function
• create a byte test vector with a non-zero value_balance, but no spends or outputs

coinbase_tx_no_joinsplit_or_spend

• copy the orchard consensus rules into the function docs - https://zips.z.cash/protocol/nu5.pdf#txnencodingandconsensus
• implement for transaction version 5:
  • create a new error variant CoinbaseHasEnableSpendsOrchard
  • if there is an orchard_shielded_data, and orchard_shielded_data.flags has ENABLE_SPENDS, return the error
  • feel free to add an orchard_shielded_data or orchard_shielded_data_flags method on Transaction
• add Orchard tests (see details below)

Each function should have tests similar to #1979:

• Transaction v5 test vectors from zcashd (or test vectors that we create and share with zcashd)
  • Orchard-only, Orchard/Sapling, Orchard/Transparent, and Orchard/Sapling/Transparent test vectors
• Arbitrary transaction v5
• an empty transaction v5, with no Orchard, Sapling, or Transparent data

We could create fake v5 transactions from our test vectors, but they are a lower priority, because they won't improve test coverage very much. (We want to focus on Orchard.)

We will verify the Orchard binding signature in a separate ticket:
#2103

Describe alternatives you've considered

These consensus rules are required for NU5.

[teor2345]

PR #2229 only implemented part of this ticket.

[mpguerra]

Oh, I didn't realise github was going to close it when #2229 merged, I was just adding the related PRs

[teor2345]

Oh, I didn't realise github was going to close it when #2229 merged, I was just adding the related PRs

Yeah it's weird, sometimes it closes them, sometimes it just leaves them alone.

I think maybe one is GitHub and the other is ZenHub?

Anyway, no big deal, we have to double-check things anyway :-)

[teor2345]

All 4 PRs for this ticket are merged, or set to auto-merge after minor fixes.