Parse default values in fields

[dtolnay]

• RFC: https://github.com/rust-lang/rfcs/pull/3681
• Tracking issue: https://github.com/rust-lang/rust/issues/132162
• Feature gate: #![feature(default_field_values)]

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