Rollup merge of rust-lang#148084 - the8472:prefixless-path-components-iter, r=Mark-Simulacrum

Optimize path components iteration on platforms that don't have prefixes

```
OLD

path::bench_path_components_iter 140.20ns/iter +/- 6.08
path::bench_path_file_name 57.62ns/iter +/- 1.62

NEW

path::bench_path_components_iter 126.74ns/iter +/- 1.46
path::bench_path_file_name 47.48ns/iter +/- 2.49
```

Author: Zalathar