Add default operator== to serializable classes

Otherwise derived classes cannot use add a default themselves. Maybe this
makes sense for more comparision operators. Note that I'm still testing the
usefulness. It already breaks when using multiple inheritane so maybe there
is a better way.

The operator is guarded by a version constraint because it is a C++20
feature.

lib/binary/serializable.h

@@ -27,6 +27,10 @@ template <typename Type, BinaryVersion v> struct BinarySerializable {
 
     static constexpr const char *qualifiedName = "ReflectiveRapidJSON::BinarySerializable";
     static constexpr auto version = v;
+
+#if __cplusplus > 201707L
+    bool operator==(const BinarySerializable<Type, v> &) const = default;
+#endif
 };
 
 template <typename Type, BinaryVersion v> inline void BinarySerializable<Type, v>::toBinary(std::ostream &outputStream, BinaryVersion version) const

lib/json/serializable.h

@@ -31,6 +31,10 @@ template <typename Type> struct JsonSerializable {
     static Type fromJson(const std::string &json, JsonDeserializationErrors *errors = nullptr);
 
     static constexpr const char *qualifiedName = "ReflectiveRapidJSON::JsonSerializable";
+
+#if __cplusplus > 201707L
+    bool operator==(const JsonSerializable<Type> &) const = default;
+#endif
 };
 
 /*!