Sprout note encryption/decryption implementation.

[hdevalence]

We need an implementation of note encryption. This has possible overlap with librustzcash.

https://zips.z.cash/protocol/protocol.pdf#sproutencrypt
https://zips.z.cash/protocol/protocol.pdf#sproutdecrypt

[dconnolly]

Two levels of completion:

All the types to represent in the transaction

Some types:

• sprout::Note (plaintext)
• sprout::OutCiphertext
• sprout::EncryptedCiphertext

And implementing methods to do the note encryption

• NoteEncryptionBuilder

But first:

• note commitment trees: Design Sprout note commitment trees. #36
• addresses
• as above, full Note implementation

Related: #266

[teor2345]

@dconnolly does this belong in the "Transaction Validation" milestone, or the "Wallet Support" milestone?

[dconnolly]

Wallet support

[teor2345]

We're not planning to do full-node wallet work any time soon.

If we do, we'll need to decide if this ticket is in scope, and update its content, because the spec has changed since it was created.