Debug output doesn't honor formatting specifier x

```rust
use num_bigint::BigUint;
 
fn main() {
    let small = 32768usize;
    let big = BigUint::from(small);
    println!("{small:x} {big:x}");
    println!("{small:x?} {big:x?}");
}
```

Prints:
```
8000 8000
8000 32768
```

It should print:
```
8000 8000
8000 8000
```