Change the serialization to CBOR

Inspired by https://github.com/TBD54566975/did-dht/pull/264

CBOR is much simpler and more efficient. There is wide support for it [across languages](https://cbor.io/impls.html).

We can also look into:

* [The Plain CBOR Representation v1.0](https://www.w3.org/TR/did-cbor-representation/) from the DID WG
* [CBOR-LD 1.0](https://json-ld.github.io/cbor-ld-spec/) from the JSON-LD org
* [dCBOR: A Deterministic CBOR Application Profile](https://datatracker.ietf.org/doc/draft-mcnally-deterministic-cbor/) representation in the IETF

I am tagging @msporny and @christophera both of whom I've spoken with about adopting CBOR. It would be great to hear your perspectives and considerations (Manu, I know you raised some great points about versioning and not conflicting with already registered terms).