Provide hint when cast needs a dereference

Provide hint when cast needs a dereference

For a given code:

```rust
vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();
```

display:

```nocode
error: casting `&f64` as `i16` is invalid
 --> foo.rs:2:35
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();
  |                              -    ^^^ cannot cast `&f64` as `i16`
  |                              |
  |                              did you mean `*s`?
```

instead of:

```nocode
error: casting `&f64` as `i16` is invalid
 --> <anon>:2:30
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect();
  |                              ^^^^^^^^
  |
  = help: cast through a raw pointer first
```