Introduced "cross-actor reference" terminology and the requirement that cross-actor references always traffic in `ConcurrentValue` types.
Introduced "cross-actor reference" terminology and the requirement that cross-actor references always traffic in `ConcurrentValue` types.

apple/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution