Returning & from a tracked function

[nikomatsakis]

I noticed that code like

#[salsa::input]
struct Input {
    #[return_ref]
    data: Vec<String>
}

#[salsa::tracked]
fn test<'db>(db: &'db dyn crate::Database, input: Input) -> &'db String {
    input.data(db).first().unwrap()
}

...will compile, but I don't think that's sound. We should be requiring the Update trait on the return value.

Filing this issue so I don't forget to think about it. =)