Consider adding sender field to notes

[bobbinth]

It may be convenient to add a sender field to notes. This field will contain account ID of the account which created the note. This field could then be used by note consumers as a reliable way determining note origin.

The sender field would need to be set in the transaction kernel at the time a note is created (i.e., via create_note procedure).

To add this field we'll need to re-define note hash and nullifier computations as follows:

note hash: hash(hash(hash(hash(serial_num, [0; 4]), script_hash), sender), vault_hash)
nullifier: hash(serial_num, script_hash, vault_hash, sender)

The main drawback of this change is that it requires one extra hash when computing a note hash (however, nullifier computation complexity remains the same).

[frisitano]

This is superseded by #46.