rustdoc-json: Report default field values information

[obi1kenobi]

RFC 3681 (tracking issue) proposes supporting default values on fields.

As I mentioned at RustWeek, I'd love to move toward having SemVer lints available in cargo-semver-checks on day 1 of new language features hitting stable Rust. This feature is a perfect candidate to trial that:

• @estebank and I have already worked to identify the new SemVer hazards in this language feature
• the cargo-semver-checks schema and lints we'd need are relatively straightforward
• all we need is for rustdoc JSON to include the info we need.

cc @aDotInTheVoid

@rustbot label A-rustdoc-json