TODO: Regenerate the marshalled repo state only when the type actually changes

[kit-ty-kate]

We could have a ppx (or do it by hand) that checks the shape of a type, hash it and only regenerate the cache when this hash changes.
This would fix the issue related to opam regenerating the state cache too often (e.g. when switching between opam dev and the latest release, with e.g. plugins)