idea: allow control of splitting depth / number of jobs

We currently have with_min_len and with_max_len to control the length of the split iterators. However, this only works on an IndexedParallelIterator that knows its length in the first place.

Perhaps we could offer different controls for the splitting depth, or the number of jobs (~2depth), which could work on any parallel iterator. This may line up more directly to how folks think about their tasks, especially if they want to split exactly into the number of threads.