The current derived debug implementation on `Witness` prints the content
field as an array of integers. We can do better than this by manually
implementing `Debug`.
With this applied `Witness` is printed as follows: (first line is `{:?}`
and the next is `{:#?}`):
Using `{:?}`:
```
Witness: { indices: 3, indices_start: 8, witnesses: [[0x00], [0x02, 0x03], [0x04, 0x05]] }
```
Using `{:#?}`:
```
Witness: {
indices: 3,
indices_start: 8,
witnesses: [
[0x00],
[0x02, 0x03],
[0x04, 0x05],
],
}
```
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