Preserve field order in with_field

### Description of new feature

I will often replace an existing structure field with some processing, e.g.:

```python
x = ak.Array(...)
x["ref"] = update_dependencies(x["ref"], guide)
...
```

At present, this re-orders the fields of the array (`"ref"` will come last). It seems reasonable/predictable that the field order would instead be preserved if we are assigning a field that already exists. 