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/DirectStep.cpp

@@ -221,6 +221,8 @@ private:
 class DirectIPaymentStep : public DirectStepI<DirectIPaymentStep>
 {
 public:
+    explicit DirectIPaymentStep() = default;
+
     using DirectStepI<DirectIPaymentStep>::DirectStepI;
     using DirectStepI<DirectIPaymentStep>::check;
 
@@ -264,6 +266,8 @@ public:
 class DirectIOfferCrossingStep : public DirectStepI<DirectIOfferCrossingStep>
 {
 public:
+    explicit DirectIOfferCrossingStep() = default;
+
     using DirectStepI<DirectIOfferCrossingStep>::DirectStepI;
     using DirectStepI<DirectIOfferCrossingStep>::check;