Fix benches: actually poll futures in benches

[kpp]

Thanks to @jonas-schievink in rust-lang/rust#70488 (comment)

Comparison from 1.42.0-stable to 1.44.0-nightly:

     Running target/release/deps/future-a45d033e4c6a8a49
future::ready/futures   time:   [1.4139 ns 1.4214 ns 1.4284 ns]                                   
                        change: [-33.075% -32.463% -31.892%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  4 (4.00%) low mild
future::ready/async_combinators                                                                             
                        time:   [1.4258 ns 1.4343 ns 1.4430 ns]
                        change: [-74.031% -73.784% -73.538%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  3 (3.00%) low mild
  3 (3.00%) high mild
  1 (1.00%) high severe

future::poll_fn/futures time:   [923.22 ps 928.82 ps 934.70 ps]                                     
                        change: [-58.380% -57.936% -57.454%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  3 (3.00%) low mild
  4 (4.00%) high mild
  1 (1.00%) high severe
future::poll_fn/async_combinators                                                                             
                        time:   [1.0679 ns 1.0721 ns 1.0772 ns]
                        change: [-57.291% -55.216% -53.284%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) high mild
  2 (2.00%) high severe

future::map/futures     time:   [1.4244 ns 1.4308 ns 1.4370 ns]                                 
                        change: [-34.310% -33.439% -32.391%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  2 (2.00%) low mild
  4 (4.00%) high mild
  6 (6.00%) high severe
future::map/async_combinators                                                                             
                        time:   [2.1948 ns 2.1974 ns 2.2002 ns]
                        change: [-72.283% -71.937% -71.663%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  4 (4.00%) high mild
  1 (1.00%) high severe

     Running target/release/deps/stream-ff2c5e87168d8d4c
stream::iter/futures    time:   [1.2016 us 1.2044 us 1.2073 us]                                  
                        change: [-43.719% -43.403% -43.102%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe
stream::iter/async_combinators                                                                             
                        time:   [1.2012 us 1.2065 us 1.2132 us]
                        change: [-44.513% -43.887% -43.455%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  1 (1.00%) high mild
  3 (3.00%) high severe

stream::next/futures    time:   [1.6517 us 1.6581 us 1.6681 us]                                  
                        change: [-21.251% -20.891% -20.513%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) high mild
  2 (2.00%) high severe
stream::next/async_combinators                                                                             
                        time:   [1.6536 us 1.6570 us 1.6607 us]
                        change: [-22.210% -21.212% -19.984%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  3 (3.00%) high mild
  6 (6.00%) high severe

stream::collect/futures time:   [3.8079 us 3.8200 us 3.8337 us]                                     
                        change: [-18.876% -18.374% -17.874%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  3 (3.00%) high mild
  2 (2.00%) high severe
stream::collect/async_combinators                                                                             
                        time:   [4.6209 us 4.6394 us 4.6621 us]
                        change: [-13.893% -13.374% -12.815%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  1 (1.00%) low mild
  2 (2.00%) high mild
  8 (8.00%) high severe

stream::map/futures     time:   [4.1590 us 4.1714 us 4.1870 us]                                 
                        change: [-5.0111% -4.2037% -3.4004%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  3 (3.00%) high mild
  8 (8.00%) high severe
stream::map/async_combinators                                                                             
                        time:   [9.2230 us 9.2599 us 9.3010 us]
                        change: [-9.8141% -8.9312% -8.1937%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  4 (4.00%) high mild
  1 (1.00%) high severe

stream::fold/futures    time:   [1.3115 us 1.3146 us 1.3179 us]                                  
                        change: [-10.420% -9.8719% -9.3034%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low mild
  5 (5.00%) high mild
  1 (1.00%) high severe
stream::fold/async_combinators                                                                             
                        time:   [2.2644 us 2.2704 us 2.2769 us]
                        change: [-49.459% -49.182% -48.925%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe