Shouldn't `Amount` handle decimals formatting with `{:.8}` ?


I would expect this to work

```
        assert_eq!("0.00000010 BTC", format!("{:.8}", Amount::from_sat(10)));
```

While it fails with:

```
  left: `"0.00000010 BTC"`,
 right: `"0.0000001 BTC"`', bitcoin/src/amount.rs:2379:9
```