internal: no std mutex

[Person-93]

This PR is to prepare for no_std support.

I've added non_poison::Mutex and Once to the platform mod.

There was once place that relied on poisoning for correctness: err_state.rs, but that didn't really need a mutex because it was guarding a Copy type, I've replaced it with a Cell.

[codspeed-hq]

Merging this PR will improve performance by 10.54%

⚠️ 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
✅ 140 untouched benchmarks
⏩ 6 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	test_empty_class_init	31.7 µs	28.7 µs	+10.54%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing Person-93:no_std_mutex (f057746) with main (58d00a1)}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Person-93]

I've moved the wip_std_feature into a separate PR and this one is blocked on that one. #6155