Replace serde with direct encoding implementation.

[cobward]

I've been looking into the canonicalisation recommendations for cbor, and realised it's going to be very difficult to implement those restrictions with serde. Also in my opinion using serde implies that you can encode and decode the data structures to any format, when really we want to just limit it to cbor. For that reason I think just implementing a to_cbor_bytes and from_cbor_bytes method on each struct would be best.

I've been looking at this crate as a good option to implement this:
https://twittner.gitlab.io/cbor-codec/cbor/index.html

The GenericDecoder appears to handle cbor canonicalisation properly, for example erroring if duplicate keys are found in a map.