try to avoid MSVC bug...

try to avoid MSVC bug (https://connect.microsoft.com/VisualStudio/feedback/details/797682/c-decltype-of-class-member-access-incompletely-implemented)

src/json.hpp

@@ -5702,7 +5702,7 @@ class basic_json
         /// the container to iterate
         basic_json& container;
         /// the type of the iterator to use while iteration
-        using json_iterator = decltype(container.begin());
+        using json_iterator = decltype(std::begin(container));
 
         /// internal iterator wrapper
         class iterator_wrapper_internal

src/json.hpp.re2c

@@ -5702,7 +5702,7 @@ class basic_json
         /// the container to iterate
         basic_json& container;
         /// the type of the iterator to use while iteration
-        using json_iterator = decltype(container.begin());
+        using json_iterator = decltype(std::begin(container));
 
         /// internal iterator wrapper
         class iterator_wrapper_internal