This gets us another 20-30% faster:

| Benchmark          | 27_pool_opts | 27_pool_cython2             |
|--------------------|--------------|-----------------------------|
| imap_unordered_seq | 897 us       | 694 us: 1.29x faster (-23%) |
| imap_unordered_par | 539 us       | 363 us: 1.49x faster (-33%) |
| imap_seq           | 1.00 ms      | 714 us: 1.41x faster (-29%) |
| imap_par           | 612 us       | 404 us: 1.52x faster (-34%) |
| map_seq            | 382 us       | 349 us: 1.09x faster (-9%)  |
| map_par            | 267 us       | 252 us: 1.06x faster (-6%)  |
| apply              | 427 us       | 406 us: 1.05x faster (-5%)  |
| spawn              | 397 us       | 360 us: 1.10x faster (-9%)  |