Extend cslib_object's get_by_field and get (by index) methods to take a custom main instance prefix

Currently both of these functions expect you to have a single "main" instance, consisting of a

For example, for this definition:
```
*create dragon_id ""
*create dragon_field_1 "name"
*create dragon_field_2 "hit_points"
*create dragon_field_max 2
```

You're expected to define:
```
*create dragon_name ""
*create dragon_hit_points ""
```

And this is where both get routines will put the relevant values, when called as normal:
```
*gosub_scene cslib_object get "dragon" 2
or
*gosub_scene cslib_object get_by_field "dragon" "name" "Locke"
```

It should be possible to add an extra parameter to both of the get functions which allows the user to specify a custom main instance with a different name, e.g.:

```
*gosub_scene cslib_object get "dragon" 2 "baby_dragon"
or
*gosub_scene cslib_object get_by_field "dragon" "name" "Locke" "baby_dragon"
```

These two examples would attempt to populate the following variables (instead of the object-named main instance):
```
*create baby_dragon_name ""
*create baby_dragon_hit_points ""
```

Of course, this name could be any valid string: giving a lot of flexibility back to the user.
For backwards compatibility (and convenience), if this last parameter isn't provide, we should still default to the main instance.