update voxel fragments lua api & replace structure.save command with fragment.save & add 'export' entry point

src/logic/scripting/lua/lua_engine.cpp

@@ -103,7 +103,7 @@ void lua::init_state(State* L, StateType stateType) {
 
     newusertype<LuaBytearray>(L);
     newusertype<LuaHeightmap>(L);
-    newusertype<LuaVoxelStructure>(L);
+    newusertype<LuaVoxelFragment>(L);
 }
 
 void lua::initialize(const EnginePaths& paths) {