Use `ArcStr` for marker values (#10453)
N.B. After fixing #10430, `ArcStr` became the fastest implementation
(and the gains were significantly reduced, down to 1-2%). See:
https://github.com/astral-sh/uv/pull/10453#issuecomment-2583344414.
## Summary
I tried out a variety of small string crates, but `Arc<str>`
outperformed them, giving a ~10% speed-up:
```console
❯ hyperfine "../arcstr lock" "../flexstr lock" "uv lock" "../arc lock" "../compact_str lock" --prepare "rm -f uv.lock" --min-runs 50 --warmup 20
Benchmark 1: ../arcstr lock
Time (mean ± σ): 304.6 ms ± 2.3 ms [User: 302.9 ms, System: 117.8 ms]
Range (min … max): 299.0 ms … 311.3 ms 50 runs
Benchmark 2: ../flexstr lock
Time (mean ± σ): 319.2 ms ± 1.7 ms [User: 317.7 ms, System: 118.2 ms]
Range (min … max): 316.8 ms … 323.3 ms 50 runs
Benchmark 3: uv lock
Time (mean ± σ): 330.6 ms ± 1.5 ms [User: 328.1 ms, System: 139.3 ms]
Range (min … max): 326.6 ms … 334.2 ms 50 runs
Benchmark 4: ../arc lock
Time (mean ± σ): 303.0 ms ± 1.2 ms [User: 301.6 ms, System: 118.4 ms]
Range (min … max): 300.3 ms … 305.3 ms 50 runs
Benchmark 5: ../compact_str lock
Time (mean ± σ): 320.4 ms ± 2.0 ms [User: 318.7 ms, System: 120.8 ms]
Range (min … max): 317.3 ms … 326.7 ms 50 runs
Summary
../arc lock ran
1.01 ± 0.01 times faster than ../arcstr lock
1.05 ± 0.01 times faster than ../flexstr lock
1.06 ± 0.01 times faster than ../compact_str lock
1.09 ± 0.01 times faster than uv lock
```
