Add NodeToNode Codec for PerasCertDiffusion

[tbagrel1]

Closes github.com/tweag/cardano-peras/issues/85.

[tbagrel1]

Given what we said in the weekly meeting today, I think this should be enough to complete ticket tweag/cardano-peras#85. Let me know if I missed something :)

[amesgen]

For really simple types like PerasRoundNo, we usually define a Serialise instance. This way, we can avoid defining new en-/decoding functions, but can rather use the generic encode/decode functions.

[amesgen]

When we serialize a self-contained entity (like a cert/block/tx/whatever), we want it to be valid CBOR by itself. Currently, this isn't valid, but rather the juxtaposition of two valid CBOR items. To make it well-formed, you can add add a encodeListLen 2. (Dually, the decoder should use decodeListLenOf 2.)

Related: We should eventually test stuff like this; I created tweag/cardano-peras#103 for that.

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Also see the validFlatTerm function for testing such stuff interactively, eg:

 Λ import Codec.CBOR.Encoding
 Λ import Codec.CBOR.FlatTerm
 Λ validFlatTerm $ toFlatTerm $ encodeWord64 123 <> encodeWord64 456
False
 Λ validFlatTerm $ toFlatTerm $ encodeListLen 2 <> encodeWord64 123 <> encodeWord64 456
True

in a cabal repl -b cborg.

[amesgen]

Instead of exposing en-/decoding functions, we could also already define a SerialiseNodeToNode instace for PerasCert (which will be required by tweag/cardano-peras#73 in any case)