Fix unit struct to serialize/deserialize in a format similar to the regular structs

[rukai]

closes https://github.com/dtolnay/serde-yaml/issues/216

I dont really know what im doing but it passes the test I wrote :shrug:

concerns:

• Do I need more specific error handling or is invalid_type fine?
• is matching on TScalarStyle::Plan and None the correct approach?
• This change to unit struct handling makes sense to me but maybe theres a reason for the old handling?
  • It looks like serde_json does the same thing as the old handling https://github.com/serde-rs/json/blob/2e1c46c89501cbe9edc41d78469ca690bf79ff65/src/de.rs#L1681