ARC: shard the weak-reference table

[DTW-Thalion]

Stacked on #399 and depends on it, so the diff currently includes #399's atomic-refcount commit; that drops out once #399 merges.

The weak-reference table used a single global lock, so weak operations on unrelated objects serialised on one mutex and scaled negatively with core count. This splits the table into address-keyed stripes, each with its own lock, so weak traffic on different objects proceeds in parallel.

Follow-up commits make the stripe count configurable (a build flag or the OBJC_WEAK_SHARD_COUNT environment variable), close a lock-selection race the sharding introduced (a use-after-free found under AddressSanitizer, reproducible with more than one shard), and make weak-slot access atomic so the sharding is also sound on weakly-ordered targets (checked under ThreadSanitizer).

Opening as a draft while #399 is in review.

[davidchisnall]

This looks more invasive than it should be. The original refactoring to make the type used for a weak ref table distinct was intended to make it easy to provide a replacement that did the sharding internally.