Disallow credentials with VC/JSON and VC/JWT members that do not encode the same value

As discussed in [PR#63](https://github.com/decentralized-identity/did-jwt-vc/pull/63/files#r583764430) there is potential ambiguity during VC/JWT <-> VC/JSON transformation if both entries are present but encode different values.

During VC or VP creation these methods should throw an error in that case and the transformations that are performed for convenience could override the entries that don't correspond to the respective encoding.

It is very inefficient to keep both representations in a credential payload, but that is an ambiguity with the spec we're working on so it must be handled.
Let's discuss the best approach before venturing a fix.