Keep old credential ID for existing credentials

In #59, we changed the format for serialized credentials to use shorter
field names for the RP and user entities.  This has an unintended side
effect:  For non-discoverable credentials that were generated with older
crate versions, the stripped data embedded into the credential ID
includes the RP and user.  If we change their serialization format, we
also change these credential IDs.

We already supported deserializing both formats using a serde alias.
This patch introduces helper enums that deserialize both formats using a
custom Deserialize implementation and keep track of the used format.
This format is then also used for serialization (using serde’s untagged
mechanism that is not available for deserialization in no-std contexts).

#59

Fixes: #111