diff: rename differ to comparer

Remove combined interface and split implementations.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

task.go

@@ -536,7 +536,7 @@ func (t *task) checkpointRWSnapshot(ctx context.Context, index *v1.Index, snapsh
 	opts := []diff.Opt{
 		diff.WithReference(fmt.Sprintf("checkpoint-rw-%s", id)),
 	}
-	rw, err := rootfs.Diff(ctx, id, t.client.SnapshotService(snapshotterName), t.client.DiffService(), opts...)
+	rw, err := rootfs.CreateDiff(ctx, id, t.client.SnapshotService(snapshotterName), t.client.DiffService(), opts...)
 	if err != nil {
 		return err
 	}