duplicate keys in CBOR set decoding #2965

[JaredCorduan]

We currently allow CBOR arrays with duplicate elements to be serialized as sets (by dropping the older duplicated elements). We also currently do not allow for using the CBOR set tag 258. In the spirit of proposed CIP-80, we consider the first issues (duplicates) a serious flaw that we should fix at the next hard fork, and we consider the second issue a non-serious flaw that we should migrate away from (so start permitting the tag at the next hard fork but do not enforce it).