Auto merge of #111781 - the8472:filter-map-chunk, r=thomcc

optimize next_chunk impls for Filter and FilterMap

```
OLD:

benchmarks:
    iter::bench_next_chunk_filter_even                 104.00ns/iter  +/- 1.00ns
    iter::bench_next_chunk_filter_map_even             101.00ns/iter  +/- 1.00ns
    iter::bench_next_chunk_filter_map_mostly_false       1.99µs/iter +/- 10.00ns
    iter::bench_next_chunk_filter_map_predictably_true  56.00ns/iter  +/- 0.00ns
    iter::bench_next_chunk_filter_mostly_false           1.15µs/iter  +/- 6.00ns
    iter::bench_next_chunk_filter_predictably_true      65.00ns/iter  +/- 1.00ns

NEW:

benchmarks:
    iter::bench_next_chunk_filter_even                  42.00ns/iter  +/- 0.00ns
    iter::bench_next_chunk_filter_map_even              49.00ns/iter  +/- 1.00ns
    iter::bench_next_chunk_filter_map_mostly_false     501.00ns/iter  +/- 3.00ns
    iter::bench_next_chunk_filter_map_predictably_true  31.00ns/iter  +/- 0.00ns
    iter::bench_next_chunk_filter_mostly_false         534.00ns/iter +/- 13.00ns
    iter::bench_next_chunk_filter_predictably_true      28.00ns/iter  +/- 1.00ns
```

Author: bors