A module that converts a type into another expression that can be used in camlp4 extensions needs to have the following functions.
val conversion : Ppxlib.Location.t ‑> field_name:string ‑> id:Ppxlib.Longident.t Ppxlib.Location.loc ‑> Ppxlib.expression
val conversion_of_type : Ppxlib.Location.t ‑> field_name:string ‑> field_ty:Ppxlib.core_type ‑> Ppxlib.expression