Automatically generate json parsing

[Nadrieril]

It's tedious to write them by hand. I found https://github.com/serde-ml/serde but it requires ocaml >= 5.1.1. We could write our own generator from rust possibly.

[sonmarcho]

Does it print nice error messages when it fails?

[Nadrieril]

I don't know I couldn't try because of ocaml version mismatches

[Nadrieril]

First big chunk was done here: https://github.com/AeneasVerif/charon/pull/283

[Nadrieril]

This is pretty much done now