Derive a note's randomness from the ephemeral shared secret.

[nventuro]

Computing the randomness as the hash of the shared secret S (see the new definition for S here: #8969) means the recipient doesn't need to be told what it is, so we can remove it from the logs and save DA costs.

Note that the key changes don't enable this: this optimization had always been available. It also doesn't change the trust assumption re. the recipient, since they're still freely choosing a 'random' value for the esk generation, even if the randomness itself is derived.

While this optimization is nice in theory, it will wreak havoc a bit with our NoteInterface, since we won't simply be able to serialize the data structure and place it in the logs: likely the logs will need to know more about how notes work in order to cooperate during log creation.