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

[the8472]

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

[rustbot]

r? @Mark-Simulacrum

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[Kobzol]

Nice! I was thinking about this when profiling rust-lang/glob#181, because it was clear that getting just the filename is unnecessary slow just becase .components() has to be called.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[the8472]

it was clear that getting just the filename is unnecessary slow just becase .components() has to be called.

added a benchmark for getting filenames