Make AtomicBool the same size as bool

[Amanieu]

This allows AtomicBool to be transmuted to a bool, which makes it more consistent with the other atomic types. Note that this now guarantees that the atomic type will always contain a valid bool value, which wasn't the case before (due to fetch_nand).

[rust-highfive]

r? @alexcrichton

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[nagisa]

The impl looks okay to me. The comment regarding CAS loop also seems to be true, and LLVM won’t accept something like atomicrmw nand i1* %2, i1 0 [ordering] anyway. I also like the cleanup made possible around every other function.

[alexcrichton]

I'd be a little nervous to land this ahead of rust-lang/rfcs#1543 and rust-lang/rfcs#1505, but it seems fine to me in principle.

I'm tagging with T-libs so this comes up during triage, but I suspect we'll merge in any case :)

[alexcrichton]

The libs team discussed this during triage yesterday, and some of our discussion led to some surprising conclusions, but in general we felt a little uncomfortable merging this ahead of rust-lang/rfcs#1543. In light of that, we're thinking that we may want to hold off on this for now until we move on one of those RFCs

[alexcrichton]

Ok, we discussed this again in light of accepting rust-lang/rfcs#1543, but we concluded that we probably don't want to do this at this time due to the uncertainty about what to do with platform-specific types. We can certainly revisit this though once the types become stable!

[Amanieu]

I don't think platform-specific types are relevant here since any platform that supports pointer-sized atomics will always support byte-sized atomics. Changing the size of AtomicBool to 1 byte will therefore not introduce any compatibility issues.