Consider misuse-resistance of the Transcript parameters

https://github.com/hdevalence/danake/pull/18#discussion_r430833214

The current API takes a `Transcript` parameter, as recommended by the Merlin API. This is good because it means that applications can add arbitrary data and are required to give a domain separator. But it also allows the potential for misuse, since if the arbitrary data is identifying, this compromises the anonymity properties of the system.