Bool CAS and Toggle don't work together about atomic

[prashantv]

Great catch! You're completely right -- CAS requires knowledge of the entire value to swap, while everything else can rely on just the last bit.

I think we have a couple of options here:

1. Don't rely on the last bit, but check the entire value. This require Toggle to become a loop that uses CAS, something like:

for {
  old := b.Load()
  if b.CAS(old, !old) {
    return true
  }
}

2. Modify CAS to do a load of the int value, and use that for an atomic.CompareAndSwap. This requires a loop in CAS though, since the API for the user should be that the swap succeeds if the old value matches, but it's possible that the old value is 3, but then a user does b.Store(true) which modifies the value, even though it's still true, the CAS would fail.

I'll send a PR for (1) since it's simpler, and I'd prefer the semantics of CAS to be a single CompareAndSwap.

cc @abhinav