Codechange: Let the compile generate the master command table out of templated command traits.

This is using a non-intrusive type-traits like templated system, which
allows compile-time validation that the command table and the command
enum match up.

src/road_cmd.cpp

@@ -37,6 +37,7 @@
 #include "genworld.h"
 #include "company_gui.h"
 #include "road_func.h"
+#include "road_cmd.h"
 
 #include "table/strings.h"
 #include "table/roadtypes.h"