clean up and finalize symmetric crypto functionality

[redshiftzero]

• [ ] Ensure we use common code paths for deriving symmetric keys for memo, note, and swap encryption (I believe we do for memo and notes): https://github.com/penumbra-zone/penumbra/issues/1265
• [ ] Add Ock type: OutgoingCipherKey: https://github.com/penumbra-zone/penumbra/issues/1267
• [ ] Generate a better name for the return value of Note::encrypt_key and add that struct
• [x] Name the derived symmetric key e,g, PayloadKey (line 138 of note.rs)
• [ ] Provide some sort of interface to get the PayloadKey - this is a more fine grained functionality than the viewing functionality since it would allow viewing a single action
• [x] Collecting the nonces together in one place (for swap, memo, note)
• [x] https://github.com/penumbra-zone/penumbra/issues/1268
• [ ] Consider encrypting the shared secret directly instead of the transmission key and ephemeral secret
• [ ] Document how the transaction key hierarchy works in the spec

Note that this is closely related to the memo change (#1222)