lint needless borrows in format args

This could probably be its own lint rather than an extension of needless_borrow (on the user facing side at least, so that we can allow/deny that specific part of the lint)

When passing args to format!() and and friends, we actually don't need to explicitly use a &borrow because we don't consume the object:

    println!("{:?} {} {}", &a, &b, &c.display());

could as well be:

    println!("{:?} {} {}", a, b, c.display());

as this code sample demonstrates:

pub fn main() {
    let a = vec!['a'];
    let b = "hello";
    let c = std::path::PathBuf::from("/");

    println!("{:?} {} {}", a, b, c.display());

    std::mem::drop(a);
    std::mem::drop(b);
    std::mem::drop(c);
}

Start by reading the existing needless_borrow lint and the format-argument examples in this issue. Determine how a separate user-facing lint should recognize unnecessary borrows in format! and related macros; done means the lint can be independently allowed or denied and covers the demonstrated cases.