Bug 909490, part 1 - Add per-zone data for use by embedder. r=billm
Bug 909490, part 2 - Add callback for zone destruction. r=billm
Bug 909490, part 3 - Add callback for zone sweeping. r=jonco
Bug 905382, part 1 - Implement a read barrier for GC things. r=jonco
Bug 905382, part 2 - Implement per-zone string conversion cache. r=bz