Consider removing `OLD_MAP_ROOT` from `set_map_item`

[PhilippGackstatter]

OLD_MAP_ROOT does not appear to be used anywhere, and this raises whether it is a useful return value from set_map_item. Callers that do need the old root could technically call get_item on the slot beforehand to fetch the current root, which may be an acceptable cost if this is used rarely.

The counter side to this is that returning the old root doesn't cost a lot, i.e. the stack space is available (at least for now), though not a strong argument by any means.

Context: #2025 (comment)

If agreed, this should be done after #2025 is merged.

[PhilippGackstatter]

As part of this, let's also address this comment, i.e. use either of:

• NEW_VALUE and OLD_VALUE
• NEW_MAP_VALUE and OLD_MAP_VALUE

I think I would slightly prefer the first one.