Please warn on "value passed in is never used" (for mutable arguments)

```
fn x() -> i32
{
    let mut a : i32 = 4;
    a = 5;
    a
}
```

warning: value assigned to `a` is never read, #[warn(unused_assignments)] on by default

```
fn y(mut a: i32) -> i32
{
    a = 3;
    a
}
```

Does not warn, even though it has pretty much the same problem. In this case, the value _passed in_ is never used.
