TagUnnamed when the value has no tag or its first tag
has no name; InvalidType when that name is not a UR type. The
reference panics at the same three points.
Throws
when toCbor() is not tagged with the first tag
(the reference reads untagged_cbor() and cannot receive such a value).
The UR of a tagged dcbor value: the type is the name of its first tag, the payload is the tag's content, as the reference's
UREncodable::ur.