Add a generic CAS loop to std::sync::Atomic*

This adds a new method to all numeric `Atomic*` types with a
safe compare-and-set loop, so users will no longer need to write
their own, except in *very* strange circumstances.

This solves #48384 with `x.fetch_max(_)`/`x.fetch_min(_)`. It
also relates to #48655 (which I misuse as tracking issue for now).

*note* This *might* need a crater run because the functions could
clash with third party extension traits.

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