replace Once-guarded UnsafeCell with AtomicPtr

[Person-93]

Removes Once from the ffi crate as discussed #6022 (comment)

[codspeed-hq]

Merging this PR will improve performance by 15.36%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 104 untouched benchmarks
⏩ 1 skipped benchmark¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	bench_pyclass_create	4.6 µs	4 µs	+15.36%

---

_{Comparing Person-93:remove_once_from_ffi (535b8f3) with main (901b032)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[davidhewitt]

Thanks!