AtomicBool does not implement Default

[thepowersgang]

The three atomic types (AtomicBool, AtomicIsize, and AtomicUsize) which include const initialisers do not also implement Default.

I believe that they should, as the presence of ATOMIC_*_INIT implies that there is an accepted default value for these three types.

[GuillaumeGomez]

I added them, I'll open a PR.