Implement ZIP 212: Allow Recipient to Derive Sapling Ephemeral Secret from Note Plaintext

[dconnolly]

Changes Sapling note plaintexts, the sending of notes, the receiving of notes, and the first byte to the new version of (0x02) of the note plaintext. After activation of the ZIP as part of Canopy activation, these changes are MUSTs.

The consensus rule about shielded coinbases will be done as #2362:

https://zips.z.cash/zip-0212

[mpguerra]

Do we need this one for our Beta release or will this be part of the wallet support work?

[dconnolly]

We did part of this but we need to do the rest to encrypt shielded notes with a wallet.

[teor2345]

We did part of this but we need to do the rest to encrypt shielded notes with a wallet.

@dconnolly can we open a separate ticket, so it's clear which parts we still need to do?

[conradoplg]

ZIP-212 was incorporated into the spec, so this should be part of #269. I'll add a note there and close this