Display implementation for IpAddr doesn't respect width or alignment

```rust
fn main() {
    println!("{:?}", format!("{:<15}", std::net::Ipv4Addr::UNSPECIFIED));
    println!("{:?}", format!("{:>15}", std::net::Ipv4Addr::UNSPECIFIED));
}
```
produces
```
"0.0.0.0"
"0.0.0.0"
```
but I'd expect
```
"0.0.0.0        "
"        0.0.0.0"
```