f3fef9a649
In Rust, the strongest guarantee that `&mut` provides is that the memory pointed to is *not aliased*, whereas `&`'s guarantees are much weaker: that the value can be aliased, and may be mutated under proper precautions (interior mutability). Our atomics though use `&mut` for mutation even while creating multiple aliases, so this changes them to use 'interior mutability', mutating through immutable references.