Adds support for the default attr to tuple variants in enums.

[tcr]

This allows the following to work:

#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub enum DOM {
    Text(
        String,
        #[serde(default, skip_serializing_if = "HashMap::is_empty")] HashMap<...>), // optional data
    ),
    Element(...),
    Comment(...),
}

DOM::Text("...", HashMap::new()) becomes encoded as an adjacently tagged enum tuple variant, but because of the skip_serializing_if parameter we may omit the second field of the variant when serializing, if the condition is true. When deserializing, at the moment, serde complains that it was expecting a tuple of two types, but found 1. The new behavior after this PR is if the default attr was used, Default::default() (or a program-supplied method) is called to populate that value.

This survives the round trip for me with both serde_json and ron.

[dtolnay]

Thanks!

[dtolnay]

Added some tests in 8b4074e and released in 1.0.82.