Add optional serde serialization and deserialization

[seritools]

As mentioned on twitter, here's the straight-forward impl of serde :)

The only thing to note is that deserialize_string is used (and visit_string implemented) instead of deserialize_str to give the possibility of directly passing through a String if the deserializer used happens to have an owned String anyways.

[bodil]

Looks good to me! 👍