Parse view types syntax

[dtolnay]

• Tracking Issue for view types rust-lang/rust#155938

fn bar(a: &mut Foo.{ a }, b: &mut Foo.{ b }) {
    a.a += 1;
    b.b += 1;
}

[scrabsha]

hi! the syntax is sort of in limbo because many people disagree on what view types should look like. we ended up going the easy route: not picking any syntax and having a view_type!() macro for now. I don't think any adjustment should be necessary in syn?

(yes, i forgot to update the description of issue #155938. i will do that now)

moreover, out of curiosity, i just noticed on the changelog of syn 3.0.0 that view types are now parsed, but i see no Type::View variant on syn's doc on docs.rs. what's happening?