renamed foreign class to entity, introduced interface class query_result_reader

test/models/airplane.hpp

@@ -8,7 +8,7 @@
 #include "matador/utils/cascade_type.hpp"
 #include "matador/utils/field_attributes.hpp"
 
-#include "matador/sql/foreign.hpp"
+#include "matador/sql/entity.hpp"
 
 #include <string>
 
@@ -21,7 +21,7 @@ struct airplane
   : id(id)
   , brand(std::move(b))
   , model(std::move(m)) {}
-  unsigned long id;
+  unsigned long id{};
   std::string brand;
   std::string model;