internal/order: add a package for ordered iteration over messages and maps

The order package replaces the mapsort and fieldsort packages.
It presents a common API for ordered iteration over message fields
and map fields.

It has a number of pre-defined orderings.

Change-Id: Ie6cd423da30b4757864c352cb04454f21fe07ee2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/239837
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/impl/codec_message.go b/internal/impl/codec_message.go
index 0e176d5..733ad9f 100644
--- a/internal/impl/codec_message.go
+++ b/internal/impl/codec_message.go
@@ -11,7 +11,7 @@
 
 	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/internal/encoding/messageset"
-	"google.golang.org/protobuf/internal/fieldsort"
+	"google.golang.org/protobuf/internal/order"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 	piface "google.golang.org/protobuf/runtime/protoiface"
 )
@@ -136,7 +136,7 @@
 		sort.Slice(mi.orderedCoderFields, func(i, j int) bool {
 			fi := fields.ByNumber(mi.orderedCoderFields[i].num)
 			fj := fields.ByNumber(mi.orderedCoderFields[j].num)
-			return fieldsort.Less(fi, fj)
+			return order.LegacyFieldOrder(fi, fj)
 		})
 	}