[ZIP 245] Transaction Identifier Digests & Signature Validation for Transparent Zcash Extensions

[daira]

This would be a straightforward extension to ZIP 244.

[daira]

Done: https://zips.z.cash/zip-0245 . Note that the ZIP number was changed; ZIP 244 is "Transaction Identifier Non-Malleability".