split meta.hpp, add detected_t (used to define concepts)

include/nlohmann/detail/conversions/from_json.hpp

@@ -15,7 +15,8 @@
 
 #include <nlohmann/detail/exceptions.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
-#include <nlohmann/detail/meta.hpp>
+#include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
 namespace nlohmann

include/nlohmann/detail/conversions/to_json.hpp

@@ -8,7 +8,8 @@
 #include <valarray> // valarray
 #include <vector> // vector
 
-#include <nlohmann/detail/meta.hpp>
+#include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 #include <nlohmann/detail/iterators/iteration_proxy.hpp>