Add `fetch_not` method on `AtomicBool` This PR adds a `fetch_not` method on `AtomicBool` performs the NOT operation on the inner value. Internally, this just calls the `fetch_xor` method with the value `true`. [See this IRLO discussion](https://internals.rust-lang.org/t/could-we-have-fetch-not-for-atomicbool-s/16881)