reduce extra includes

src/data/StructLayout.cpp

@@ -6,6 +6,7 @@
 #include <string.h>
 #include <algorithm>
 
+#include "data/dv.hpp"
 #include "util/data_io.hpp"
 #include "util/stringutil.hpp"
 

src/data/dv_fwd.hpp

@@ -0,0 +1,6 @@
+#pragma once
+
+namespace dv {
+    class value;
+    struct optionalvalue;
+}