Prevent accidental aggregates

*  The compiler can provide many non-explicit constructors for
   aggregate types.  This is sometimes desired, but it can
   happen accidentally, resulting in run-time errors.

*  This commit assures that no types are aggregates unless existing
   code is using aggregate initialization.

src/ripple/protocol/STObject.h

@@ -251,6 +251,8 @@ private:
 
     struct Transform
     {
+        explicit Transform() = default;
+
         using argument_type = detail::STVar;
         using result_type = STBase;