better split_into_n for chunkdivision

[stevengj]

It would be nice to replace the split_into_three function from #976 into a similar routine that splits into n for any n. Basically you just call find_best_split(n, ...), then find_best_split(n-1, ...) on the remaining chunk, then find_best_split(n-2, ...), and so on n-1 times.

This would also let us handle other prime factors (e.g. 5) more efficiently.