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/app/paths/impl/XRPEndpointStep.cpp

@@ -167,6 +167,8 @@ private:
 class XRPEndpointPaymentStep : public XRPEndpointStep<XRPEndpointPaymentStep>
 {
 public:
+    explicit XRPEndpointPaymentStep() = default;
+
     using XRPEndpointStep<XRPEndpointPaymentStep>::XRPEndpointStep;
 
     XRPAmount