`AtomicRefCell.borrow_mut()` sometimes panics

[magnet]

I experienced a panic with AtomicRefCell.borrow_mut() :

thread '<unnamed>' panicked at 'already mutably borrowed', /home/magnet/.cargo/registry/src/github.com-1ecc6299db9ec823/atomic_refcell-0.1.3/src/lib.rs:208:9

I could not reproduce so there is a problem there. I'll replace with parking_lot::Mutex in the next release. There are many more writers than readers so a RwLock would not help significantly. To make writing less contended, another approach will be required.