Consider whether CBOR tags should be mapped to/from Ion annotations

[tgregg]

IonReaderFromCbor/IonWriterToCbor both currently do nothing with CBOR tags. On read, non-zero tags are dropped, and on conversion from Ion, no attempt is made to tunnel Ion annotations via CBOR tags.

Since CBOR tags are numeric with implicit meaning, while Ion annotations are user-provided text included in the stream, it's not clear how this mapping would be performed in a way that would actually be useful in the real world. I'm opening this issue as a place for further discussion on this if necessary.