Especially for map. None of the pools really need map to go through
imap since they have to wait for everything anyway and they return
results ordererd.

| Benchmark          | 36_threadpool_master | 36_threadpool_opt_cond5     |
|--------------------|----------------------|-----------------------------|
| imap_unordered_seq | 1.15 ms              | 1.07 ms: 1.08x faster (-7%) |
| imap_unordered_par | 1.02 ms              | 950 us: 1.08x faster (-7%)  |
| imap_seq           | 1.17 ms              | 1.10 ms: 1.06x faster (-6%) |
| imap_par           | 1.07 ms              | 1000 us: 1.07x faster (-7%) |
| map_seq            | 1.16 ms              | 724 us: 1.60x faster (-37%) |
| map_par            | 1.07 ms              | 646 us: 1.66x faster (-40%) |
| apply              | 1.22 ms              | 1.14 ms: 1.07x faster (-7%) |
| spawn              | 1.21 ms              | 1.13 ms: 1.07x faster (-7%) |