Rollup merge of #79285 - yoshuawuyts:stabilize-arc_mutate_strong_count, r=m-ou-se
Stabilize Arc::{increment,decrement}_strong_count
Tracking issue: https://github.com/rust-lang/rust/issues/71983
Stabilizes `Arc::{incr,decr}_strong_count`, enabling unsafely incrementing an decrementing the Arc strong count directly with fewer gotchas. This API was first introduced on nightly six months ago, and has not seen any changes since. The initial PR showed two existing pieces of code that would benefit from this API, and included a change inside the stdlib to use this.
Given the small surface area, predictable use, and no changes since introduction, I'd like to propose we stabilize this.
closes https://github.com/rust-lang/rust/issues/71983
r? `@Mark-Simulacrum`
## Links
* [Initial implementation](https://github.com/rust-lang/rust/pull/70733)
* [Motivation from #68700](https://github.com/rust-lang/rust/pull/68700#discussion_r396169064)
* [Real world example in an executor](https://docs.rs/extreme/666.666.666666/src/extreme/lib.rs.html#13)
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