Deny unsafe operations in unsafe fns in libstd/sync/

[poliorcetics]

Partial fix of #73904.

This encloses unsafe operations in unsafe fn in libstd/sync/.

@rustbot modify labels: F-unsafe-block-in-unsafe-fn

[rust-highfive]

r? @hanna-kruppe

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[Mark-Simulacrum]

I suspect that fixing these may not be worth it -- they look like they're all related to the mpsc module which is essentially abandoned at this point (and there's ongoing experiments in the ecosystem which are looking to replace it).

[hanna-kruppe]

r? @Mark-Simulacrum

[Mark-Simulacrum]

@poliorcetics Would you be opposed to just closing this? This is currently not quite in the shape that I'd like it to be for merging it and I feel like it would be better to dedicate time elsewhere -- editing the mpsc code is pretty much not viable today due to lack of good reviewers, and so I'd rather not spend time hashing out good safety annotations.

[poliorcetics]

No problems on my side, go ahead.

[LeSeulArtichaut]

@Mark-Simulacrum I think we should have a PR adding just the unsafe blocks then? @polioretics would you mind filing a PR for that?

[Mark-Simulacrum]

I don't think there's any point to doing so, we can just allow the lint for this module.