Spawn one task per thread in `_combine_rows_with_first!` so that custom grouped
operations that return a single row are run in parallel. This is optimal if
operations take about the same time for all groups. Spawning one task per group
could be faster if these times vary a lot, but the overhead would be larger:
we could add this as an option later.

The implementation is somewhat tricky as output columns need to be reallocated
when a new return type is detected.