Use Arc<str> for marker values

[charliermarsh]

Summary

I tried out a variety of small string crates, but Arc<str> outperformed them, giving a ~10% speed-up:

❯ 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

[zanieb]

Nice and simple

[konstin]

Which test case did you use?