Generalize serialization/deserialization of YAML nodes

[fktn-k]

To generalize serialization/deserialization features of YAML nodes, those features are wrapped as basic_node class APIs: deserialize() & serialize().
Internal operations are still executed by basic_serializer/basic_deserializer class objects.
This will enable us to improve internal operations in serialization/deserialization features while keeping backward compatibility of those APIs.

[fktn-k]

Completed in #134.