feat(serde): rewrite with a simpler model

[leostera]

This PR rewrites Serde to use a much simpler model that:

• has no intermediate representation of data
• supports direct data-to/from-format transformations
• is actually usable without a ppx
• fits in a single module

On top of this, we've rewritten the JSON format module and managed to get:

• unit variants
• newtype variants
• tuple variants
• record variants
• records
• nested records
• options
• lists
• arrays

and have roundtrip tests for all of them.