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/impl/SField.cpp

@@ -46,6 +46,8 @@ using StaticScopedLockType = std::lock_guard <std::mutex>;
 // Give this translation unit only, permission to construct SFields
 struct SField::make
 {
+    explicit make() = default;
+
     template <class ...Args>
     static SField one(SField const* p, Args&& ...args)
     {