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/ErrorCodes.cpp

@@ -29,6 +29,8 @@ namespace std {
 template <>
 struct hash <ripple::error_code_i>
 {
+    explicit hash() = default;
+
     std::size_t operator() (ripple::error_code_i value) const
     {
         return value;