unused_mut lint improvement: warn when variable only copied into closure

Minimal example of the lint failure to fire:
```
fn foo() -> bool {
    let mut res = false;
    
    let mut do_thing = move || res = true;
    do_thing();
    
    res
}
```

`foo` unconditionally returns false, but the top-level binding of `res` doesn't get an `unused_mut` warning.