Initialize rayon lazily (#9435)
When performing a noop sync, we don't need the rayon threadpool, yet we
pay for its initialization:
Be making the initialization lazy, we avoid that cost:
This code runs every time before user code in `uv run`.
This means that before calling rayon, one now needs to call
`LazyLock::force(&RAYON_INITIALIZE);`.
Performance mode (CPU 0 is a perf core):
```
$ taskset -c 0 hyperfine --warmup 5 -N "/home/konsti/projects/uv/uv-main sync" "/home/konsti/projects/uv/target/profiling/uv sync"
Benchmark 1: /home/konsti/projects/uv/uv-main sync
Time (mean ± σ): 4.5 ms ± 0.1 ms [User: 2.7 ms, System: 1.8 ms]
Range (min … max): 4.4 ms … 6.4 ms 640 runs
Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.
Benchmark 2: /home/konsti/projects/uv/target/profiling/uv sync
Time (mean ± σ): 4.4 ms ± 0.1 ms [User: 2.7 ms, System: 1.6 ms]
Range (min … max): 4.3 ms … 5.0 ms 679 runs
Summary
/home/konsti/projects/uv/target/profiling/uv sync ran
1.03 ± 0.04 times faster than /home/konsti/projects/uv/uv-main sync
```
Power saver mode:
```
$ hyperfine --warmup 5 -N "/home/konsti/projects/uv/uv-main sync" "/home/konsti/projects/uv/target/profiling/uv sync"
Benchmark 1: /home/konsti/projects/uv/uv-main sync
Time (mean ± σ): 28.1 ms ± 1.2 ms [User: 15.5 ms, System: 20.3 ms]
Range (min … max): 25.7 ms … 31.9 ms 102 runs
Benchmark 2: /home/konsti/projects/uv/target/profiling/uv sync
Time (mean ± σ): 24.0 ms ± 1.2 ms [User: 13.8 ms, System: 9.9 ms]
Range (min … max): 22.2 ms … 28.2 ms 122 runs
Summary
/home/konsti/projects/uv/target/profiling/uv sync ran
1.17 ± 0.08 times faster than /home/konsti/projects/uv/uv-main sync
```
