support records/string references

I often want to encode a bunch of identically-shaped objects like

{"name": "raylu", "id": 1}
{"name": "bob", "id": 2}

msgpackr supports "records" for this: https://github.com/kriszyp/msgpackr#record--object-structures

cbor2 supports "string references": https://cbor2.readthedocs.io/en/latest/usage.html#string-references

it would be nice if msgpack-rust (and, thus, [ormsgpack](https://github.com/aviramha/ormsgpack)) supported something like either of these