Support simple Maps additionally to Records for unregistered claims

[gaku-sei]

Not all unregistered claims have their attribute values known at compile time, adding support to Maps would allow for more dynamic claims

[gaku-sei]

I might release 0.2.0 first, this will wait until 0.2.1

[jim108dev]

Hi! Can the unregistered claims not made to adhere to ReadForeign and WriteForeign instead of Decodable, Encodable? This way other frontends/technologies could process the information stored in a token. As of now, if you use a Maybe type it cannot be read by an Angular frontend for example.