Auto merge of #86898 - the8472:path-cmp, r=dtolnay

Auto merge of #86898 - the8472:path-cmp, r=dtolnay

Add fast path for Path::cmp that skips over long shared prefixes

```
# before
test path::tests::bench_path_cmp_fast_path_buf_sort               ... bench:      60,811 ns/iter (+/- 865)
test path::tests::bench_path_cmp_fast_path_long                   ... bench:       6,459 ns/iter (+/- 275)
test path::tests::bench_path_cmp_fast_path_short                  ... bench:       1,777 ns/iter (+/- 34)

# after
test path::tests::bench_path_cmp_fast_path_buf_sort               ... bench:      38,140 ns/iter (+/- 211)
test path::tests::bench_path_cmp_fast_path_long                   ... bench:       1,471 ns/iter (+/- 24)
test path::tests::bench_path_cmp_fast_path_short                  ... bench:       1,106 ns/iter (+/- 9)
```