Allow chunks that are larger than the maximum chunk size in index construction

When incrementally constructing a compact/ordinary index and the max chunk size is exceeded, then currently we might split up the output into multiple chunks if the serialised form of the index has a size that is mutiple of the max chunk size. It might be worth it to just return a larger chunk in that case (maybe still a multiple of the max chunk size), since we don't really rely on chunks being a specific size anywhere in the `RunBuilder`/`RunAcc` code