ord: generated comparison function traverses record fields in reverse

[ygrek]

witness :

# #require "ppx_deriving.std";;
# type t = { a : int; b : int } [@@deriving ord];;
type t = { a : int; b : int; }
val compare : t -> t -> Ppx_deriving_runtime.int = <fun>
# compare {a=1;b=2} {a=2;b=1};;
- : Ppx_deriving_runtime.int = 1

$ ocamlfind ocamlc -dsource -package ppx_deriving.std q.ml
type t = {
  a: int ;
  b: int }[@@deriving ord]
let rec (compare : t -> t -> Ppx_deriving_runtime.int) =
  ((let open! Ppx_deriving_runtime in
      fun lhs  ->
        fun rhs  ->
          match Pervasives.compare lhs.b rhs.b with
          | 0 -> Pervasives.compare lhs.a rhs.a
          | x -> x)
  [@ocaml.warning "-A"])