Adopt external tagging to serialized union values

[serde][] which is Rust's serialization/deserialization library give a option to [represent enum][enum-repr]. A deserializer doesn't need to read a whole JSON to know which tag is to deserialize if Nirum has a similar feature for Union type.

[serde]: https://serde.rs/
[enum-repr]: https://serde.rs/enum-representations.html