Add the members of a diff operation without looking them up

diff_operation() created each operation through three operator[] calls and
assignments. The keys "op", "path" and "value" are distinct and already in
the order of std::map, so they are added with the positional insert that
flatten() uses, constructing the values in place.

diff() (instructions on macOS, Linux Ir GCC 13 / Clang 18): canada 85.3M -> 82.0M,
51.1M -> 50.7M / 55.3M -> 50.2M; citm_catalog 22.0M -> 21.5M, 14.3M -> 14.2M /
14.9M -> 14.2M; twitter 12.4M -> 12.0M, 7.5M unchanged / 8.0M -> 7.5M.
Output is identical.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -39899,13 +39899,18 @@ public:
     /// @brief append an operation @a op for @a path to @a result, with @a value if it is not null
     static void diff_operation(basic_json& result, const char* op, const string_t& path, const basic_json* value)
     {
+        // the keys are distinct and in the order of std::map; the members are
+        // added without looking them up
         basic_json entry(value_t::object);
-        entry["op"] = op;
-        entry["path"] = path;
+        auto& members = *entry.m_data.m_value.object;
+        typename object_t::iterator hint = members.end();
+        json_pointer::add_flattened(members, hint, string_t("op"), string_t(op), detail::priority_tag<2> {});
+        json_pointer::add_flattened(members, hint, string_t("path"), path, detail::priority_tag<2> {});
         if (value != nullptr)
         {
-            entry["value"] = *value;
+            json_pointer::add_flattened(members, hint, string_t("value"), *value, detail::priority_tag<2> {});
         }
+        entry.set_parents();
         result.push_back(std::move(entry));
     }