AtomicOption is not pointer sized

[brson]

AtomicOption is an atomic, nullable unique pointer, but because it has a dtor it does not have pointer size:

rust: ~"size_of::<AtomicOption<()>>(): 16"
rust: ~"size_of::<uint>(): 8"

It would be very convenient if this type was freely castable to pointer-size types.

[thestinger]

I have #7138 open about potentially adding an attribute to omit the drop flag by zeroing the struct instead and just calling the destructor multiple times - letting the Drop implementation deal with it. Seems like it would be usable for this too.