User-defined witnesses in Json module are shadowed

[craigfe]

Because of the global open of Irmin.Type in the deriver, any user-defined witnesses in a Json module will be shadowed by the Irmin.Type.Json module.

module Json = struct
  include Yojson.Safe
  let t = Irmin.Type.(map string) from_string to_string
end

type bench_data = {
  uuid: Uuidm.t;
  data: Json.t
} [@@deriving irmin]

gives

32 | ..type bench_data = {
33 |     uuid: Uuidm.t;
34 |     data: Json.t
35 |   } [@@deriving irmin]
Error: Unbound value Json.t