Implement Transparent and Sapling validation for transaction v5

[teor2345]

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

In #1829 and #1980, we parse and check transparent and sapling in transaction v5. But we need to call those functions in the transaction verifier.

Describe the solution you'd like

add Transaction::V5 to transaction::check (#1980 + PRs, #2070):

• quote the relevant consensus rules in each function's docs
• add orchard TODOs
• refactor to remove duplicate code, and make the consensus rule logic clearer
• implement the missing "no PrevOuts" coinbase check, added a few days ago in zcash/zips@10710d9

add Transaction::V5 to transaction::Verifier:

• reject V5 transactions before the NU5 activation height on the configured network
• make sure Transaction.network_upgrade matches the currently active network upgrade (Validate nConsensusBranchId #2100)
  • the current upgrade is based on the height and configured network, it won't always be NU5

Other transparent and sapling consensus rules:

• quote the relevant consensus rules in each function's docs
• add orchard TODOs
• refactor to remove duplicate code, and make the consensus rule logic clearer

Describe alternatives you've considered

These consensus rules are required for NU5.

Additional context

We also need to implement Orchard validation for transaction v5 (#2379)