xarc 0.3.0

`xarc` provides atomically swappable atomically refcounted smart pointers as a safer building block for lockfree algorithms than raw atomic pointers. `Xarc` is comparable to `Arc` but with the additional ability to atomically be swapped into and out of `AtomicXarc`. `Xarc` is dereferenceable but cannot have its contents atomically swapped. `AtomicXarc` can have its contents atomically swapped but is not dereferenceable.