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/misc/impl/AmendmentTable.cpp

@@ -91,7 +91,7 @@ struct AmendmentState
     /** The name of this amendment, possibly empty. */
     std::string name;
 
-    AmendmentState () = default;
+    explicit AmendmentState () = default;
 };
 
 /** The status of all amendments requested in a given window. */