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

[pickfire]

Is this E-hard?

[oli-obk]

probably not, but it requires some gray matter wrangling. For this to be doable in a reasonable matter, we likely need to implement it on MIR and use dataflow to track mutable references.

[GnomedDev]

Isn't this a duplicate of #353?