Parse default values in fields

[dtolnay]

• RFC: [RFC] Default field values rust-lang/rfcs#3681
• Tracking issue: Tracking issue for RFC 3681: Default field values rust-lang/rust#132162
• Feature gate: #![feature(default_field_values)]

#[derive(Default)]
struct Pet {
    name: Option<String>,
    age: i128 = 42,
    //        ^^^^
}