Consider emitting a warning if a negative constant with no suffix is silently converted to unsigned

In the following function, `-1` is silently converted to `std::u32::max_value`, which makes it hard to figure out why the expression `-1 < x` is false without looking up the type of `x`.

``` rust
fn main() {
    let x = 42u32;
    println!("{}", -1 < x)
}
```
