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