Use std::iterator_traits to extract iterator_category (nlohmann#3544)

* Use `std::iterator_traits` to extract `iterator_category`

In third-party STL implementations, `array_t::iterator` might be a pointer (e.g., `vector` in [EASTL](https://github.com/electronicarts/EASTL)) rather than a class, in which case directly using `array_t::iterator::iterator_category` is invalid. This commit fixes it with `std::iterator_traits`, which handles pointers correctly.

* add the changes to the single-header version

Author: Mike-Leo-Smith

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -53,7 +53,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
                   "iter_impl only accepts (const) basic_json");
     // superficial check for the LegacyBidirectionalIterator named requirement
     static_assert(std::is_base_of<std::bidirectional_iterator_tag, std::bidirectional_iterator_tag>::value
-                  &&  std::is_base_of<std::bidirectional_iterator_tag, typename array_t::iterator::iterator_category>::value,
+                  &&  std::is_base_of<std::bidirectional_iterator_tag, typename std::iterator_traits<typename array_t::iterator>::iterator_category>::value,
                   "basic_json iterator assumes array and object type iterators satisfy the LegacyBidirectionalIterator named requirement.");
 
   public:

single_include/nlohmann/json.hpp

@@ -12393,7 +12393,7 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
                   "iter_impl only accepts (const) basic_json");
     // superficial check for the LegacyBidirectionalIterator named requirement
     static_assert(std::is_base_of<std::bidirectional_iterator_tag, std::bidirectional_iterator_tag>::value
-                  &&  std::is_base_of<std::bidirectional_iterator_tag, typename array_t::iterator::iterator_category>::value,
+                  &&  std::is_base_of<std::bidirectional_iterator_tag, typename std::iterator_traits<typename array_t::iterator>::iterator_category>::value,
                   "basic_json iterator assumes array and object type iterators satisfy the LegacyBidirectionalIterator named requirement.");
 
   public: