AtomicBitSet Debug crashes

[funnisimo]

This crashes...

use hibitset::AtomicBitSet;

fn main() {
    let bitset = AtomicBitSet::new();
    println!("bitset = {:?}", bitset);
}

The proposed fix implements Debug for AtomicBlock so that the error doesn't happen - please check the output to make sure it is what you want it to be for internal debugging. It also implements AtomicBitSet Debug manually to be the output of iter() - which I felt was more helpful to users of the library.