AtomicBorrow race condition

[zakarumych]

In src/borrow.rs functions AtomicBorrow::borrow and AtomicBorrow::release_mut conflict with each other.

This execution order will break internal state

Atomic value is 0 originally.
Thread #1 AtomicBorrow::borrow_mut() stores UNIQUE_BIT in atomic value.
Thread #2 AtomicBorrow::borrow() increments atomic value. Sees UNIQUE_BIT set
Thread #1 AtomicBorrow::release_mut() stores 0 in atomic value.
Thread #2 AtomicBorrow::borrow() decrements atomic value.
Atomic value is usize::MAX.

[zakarumych]

Solution:
Rewrite AtomicBorrow::release_mut() implementation as:

let value = self.0.fetch_and(!UNIQUE_BIT, Ordering::Release);
debug_assert_ne!(value & UNIQUE_BIT, 0);

[Ralith]

Thanks for finding this!