Draft: Encodings for the `Decode` module of the Webassembly interpreters

Context

Me: Let’s write Data_encodings for the _kont types of Decode, even if they will be replaced by tree encoders/decoders eventually. This way, we will be able to at least have a PVM that can parse the boot sector. Also me: Oh no, they are already using chunked/lazy data, and there are no tree encoders for those.

Hence the draft mode. Note sure what to do with this. Some encodings will remain, so it’s not all lost.

Manually testing the MR

Checklist

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