Lint needless `&mut` when `&` suffices

[phil-opp]

Idea: Add a lint for unneeded mutable borrows when an immutable borrow suffices:

fn main() {
    let m = &mut 21; // `&21` would suffice
    println!("{}", m);
}

This lint could also catch an unneeded ref mut when a plain ref suffices:

fn main() {
    let test = &mut Test{a: 1};
    let &mut Test{ ref mut a } = test;

    println!("{}", a);
}

struct Test {
    a: i32,
}

cc @oli-obk