AtomicArc via coalesced deferred RC, resolves #160

[acsearle]

Another draft of a (potentially) fast, (arguably) ergonomic AtomicReference-like type that leverages crossbeam's collector (and requires some minimal additional hooks into it.)

• Atomic operations are protected by a crossbeam epoch guard
• Pending increments and decrements are written to a thread-local ledger
• Pending increments and decrements will often cancel within the ledger and never touch the atomic reference count
• At the end of the guarded region, the increments are applied immediately and the decrements are deferred to the garbage collector
• Loads and failed compare_exchanges produce non-owning 'borrowed' objects with lifetimes limited to the guard, that don't need to touch even the thread local ledger
• Therefore, loads are pure atomic reads, and 'as fast as possible'?
• Compare exchange loops only generate a hit to the atomic reference count when successful, and even that is deferred