Restore contacts from backup

XXClient library

• Improve error handling in UdMultiLookupCallback.

xx-messenger example

• Improve backup restoration.
• Restore contacts.
• Add contact lookup feature.
• Display contact ids.
• Improve sending requests.
• Restore username from restored facts, not from restored params.

Examples/xx-messenger/Sources/AppCore/SharedUI/Data+hexString.swift

+import Foundation
+
+extension Data {
+  public var hexString: String {
+    map { String(format: "%02hhx ", $0) }.joined()
+  }
+}