Clean json generation

[N1ark]

This PR sets up the codebase to enable the addition of a new serialisation format, using serde's postcard format.

This cleans up a lot of ad-hoc behaviours in serialisation: the crate's type is now entirely derived, and it's deserialiser entirely generated too, with only a tiny wrapper to get a more nicely shaped crate out of it.

More precisely:

• Split generate-ml/main.rs (previously 1400 lines) into several modules: to_ocaml_ty for the generation of OCaml types, of_json for the generation of the JSON deserializer, and util.
• Add Generated_FullAst.ml (exposed in GAst.ml), which contains the "root" types of the AST that weren't generated before: translated_crate and body (and I also moved some related types in there)
• Merged all _OfJson files, to simplify translation
• Merged all Print_ files, to also simplify things

I tried splitting the commits to be as reviewable as possible; note that OCaml does not compile from commit 4 (Add FullAst) to commit 10 (Merge all OfJson), since making everything compile in one go would have been too big of a commit.

ci: use AeneasVerif/aeneas#959
ci: use AeneasVerif/eurydice#397

[N1ark]

hmm the OCaml tests pass for me locally, not sure how this fails given files should be deserialized first? could it be Nix somehow reusing old .llbc files or not re-building something...? i forgot to push a change :p

if you're happy with this I'll update Aeneas and Eurydice

[Nadrieril]

That's so much better :') Looks great to me

[Nadrieril]

No need to fix this but fyi self.current_item.as_deref() == Some("translated_crate") exists :)

[Nadrieril]

Let's goooo