Add Listen functor

Sources/ElixxirDAppsSDK/Functors/Listen.swift

+import Bindings
+import XCTestDynamicOverlay
+
+public struct Listen {
+  public var run: (Int, String, SingleUseCallback) throws -> Cancellable
+
+  public func callAsFunction(
+    e2eId: Int,
+    tag: String,
+    callback: SingleUseCallback
+  ) throws -> Cancellable {
+    try run(e2eId, tag, callback)
+  }
+}
+
+extension Listen {
+  public static let live = Listen { e2eId, tag, callback in
+    var error: NSError?
+    let stopper = BindingsListen(
+      e2eId,
+      tag,
+      callback.makeBindingsSingleUseCallback(),
+      &error
+    )
+    if let error = error {
+      throw error
+    }
+    guard let stopper = stopper else {
+      fatalError("BindingsListen returned `nil` without providing error")
+    }
+    return Cancellable {
+      stopper.stop()
+    }
+  }
+}
+
+extension Listen {
+  public static let unimplemented = Listen(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}