Add ContactIdProvider

Sources/ElixxirDAppsSDK/ContactIdProvider.swift

+import Bindings
+
+public struct ContactIdProvider {
+  public var get: (Data) throws -> Data
+
+  public func callAsFunction(contact: Data) throws -> Data {
+    try get(contact)
+  }
+}
+
+extension ContactIdProvider {
+  public static let live = ContactIdProvider { contact in
+    var error: NSError?
+    let id = BindingsGetIDFromContact(contact, &error)
+    if let error = error {
+      throw error
+    }
+    guard let id = id else {
+      fatalError("BindingsGetIDFromContact returned `nil` without providing error")
+    }
+    return id
+  }
+}
+
+#if DEBUG
+extension ContactIdProvider {
+  public static let failing = ContactIdProvider { _ in
+    struct NotImplemented: Error {}
+    throw NotImplemented()
+  }
+}
+#endif