atomptr vs xarc vs orx-pinned-concurrent-col

A safe copy-on-write wrapper around `AtomicPtr` with some extra tricks

`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.

A core data structure with a focus to enable high performance, possibly lock-free, concurrent collections using a PinnedVec as the underlying storage.