Update StringOps.* implementation

[xuwei-k]

• xuwei-k/string-repeat-benchmark@330544d
• https://github.com/xuwei-k/string-repeat-benchmark/actions/runs/20405673816/job/58635011105#step:5:374
• https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/String.html#repeat(int)

[info] Benchmark     (size)   Mode  Cnt           Score        Error  Units
[info] Main.newImpl       1  thrpt   10  1606571321.905 ± 992031.056  ops/s
[info] Main.newImpl      10  thrpt   10    43216576.705 ±  29480.044  ops/s
[info] Main.newImpl     100  thrpt   10    23027077.051 ±  87609.285  ops/s
[info] Main.newImpl    1000  thrpt   10     3877430.815 ±  56856.895  ops/s
[info] Main.oldImpl       1  thrpt   10    80238683.180 ± 230581.751  ops/s
[info] Main.oldImpl      10  thrpt   10    39374657.631 ± 118702.198  ops/s
[info] Main.oldImpl     100  thrpt   10     5530021.313 ±  15677.695  ops/s
[info] Main.oldImpl    1000  thrpt   10      525098.038 ±   1315.209  ops/s