[ByteString] Update ByteString.cString implementation.
[ByteString] Update ByteString.cString implementation.

 - We now have a reasonable stdlib method for implementing this.

abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager