Add function `Arc/Rc::as_weak(…)` to convert `&Arc/Rc` to `&Weak`#100472

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jeremyBanks wants to merge 7 commits intorust-lang:masterfrom jeremyBanks:rc_as_weak

Commits on Aug 16, 2022

as_weak: add a SAFETY comment and mention the potentially-weird weak_count() result in the documentation
jeremyBanks
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