Implement windowsDiff.Compare via hcsshim/pkg/ociwclayer

This parallels the implementation of windowsDiff.Apply, including
bouncing very briefly though archive.WriteDiff and then straight back
out into Windows-specific code.

It's mostly pulling existing mechanisms from non-Windows Compare or
Windows Apply, and highlights that there's probably a lot of scope for
refactoring on top of this.

Now the export-related integration tests pass CI on Windows.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

integration/client/export_test.go

@@ -20,7 +20,6 @@ import (
 	"archive/tar"
 	"bytes"
 	"io"
-	"runtime"
 	"testing"
 
 	. "github.com/containerd/containerd"
@@ -30,8 +29,7 @@ import (
 
 // TestExport exports testImage as a tar stream
 func TestExport(t *testing.T) {
-	// TODO: support windows
-	if testing.Short() || runtime.GOOS == "windows" {
+	if testing.Short() {
 		t.Skip()
 	}
 	ctx, cancel := testContext(t)