PersistentHugr's `HugrView::children` does not guarantee ordering

As of merging https://github.com/CQCL/hugr/pull/2595:

Beyond the first two children, the order of

```rust
persistent_hugr.children(node)
```

and

```rust
let hugr = persistent_hugr.to_hugr();
hugr.children(node);
```

may not match when `node` is a Dataflow parent. AFAIK this should not lead to observable differences according to current specs, but might be an issue at some point.