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/basics/qalloc.h

@@ -111,6 +111,8 @@ public:
     template <class U>
     struct rebind
     {
+        explicit rebind() = default;
+
         using other = qalloc_type<U, ShareOnCopy>;
     };