fix and improve module (#3386)

* export public documented API
* don't export `namespace detail`
* add `std.h` into module
* add missing namespace qualification in `xchar.h`
* fix call to `detail::get_iterator` in `xchar.h`
* fix ambiguous overload of `detail::isfinite` in `chrono.h`

include/fmt/args.h

@@ -59,6 +59,7 @@ class dynamic_arg_list {
   }
 };
 }  // namespace detail
+FMT_BEGIN_EXPORT
 
 /**
   \rst
@@ -229,6 +230,7 @@ class dynamic_format_arg_store
   }
 };
 
+FMT_END_EXPORT
 FMT_END_NAMESPACE
 
 #endif  // FMT_ARGS_H_