`MacAddr` does not respect string format padding

Consider:

```rust
format!("{:<20}", macaddr::MacAddr6::new(1,2,3,4,5,6))
```

This yields:

```rust
"01:02:03:04:05:06"
```

If we instead convert to a String first:

```rust
format!("{:<20}", macaddr::MacAddr6::new(1,2,3,4,5,6).to_string())
```

We get:

```
"01:02:03:04:05:06   "
```