Add race, timeout, and nilTimeout to Guarantee

This adds the following methods to `Guarantee`:

- `Guarantee.race`, a static method. It's like `Promise.race` but it
  returns a Guarantee, not a Promise. (To match `Promise.race`, there
  are actually two versions of this method: one that takes an array of
  Guarantees and one that takes varargs.)
- `timeout`, an instance method. It's like Promise's `timeout` but it
  returns a Guarantee.
- `nilTimeout`, an instance method. It's like Promise's `nilTimeout` but
  it returns a Guarantee.

SignalCoreKit/src/Promises/Guarantee+Race.swift

@@ -0,0 +1,31 @@
+//
+// Copyright 2023 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+//
+
+import Foundation
+
+public extension Guarantee {
+    static func race<T>(
+        on scheduler: Scheduler,
+        _ guarantees: Guarantee<T>...
+    ) -> Guarantee<T> {
+        return race(on: scheduler, guarantees)
+    }
+
+    static func race<T>(
+        on scheduler: Scheduler,
+        _ guarantees: [Guarantee<T>]
+    ) -> Guarantee<T> {
+        let (result, future) = Guarantee<T>.pending()
+
+        for guarantee in guarantees {
+            guarantee.done(on: scheduler) { result in
+                guard !future.isSealed else { return }
+                future.resolve(result)
+            }
+        }
+
+        return result
+    }
+}

SignalCoreKit/src/Promises/Guarantee+Timeout.swift

@@ -0,0 +1,27 @@
+//
+// Copyright 2023 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+//
+
+import Foundation
+
+public extension Guarantee {
+    func nilTimeout(
+        on scheduler: Scheduler,
+        seconds: TimeInterval
+    ) -> Guarantee<Value?> {
+        let withOptionalValue: Guarantee<Value?> = self.map(on: scheduler) { $0 }
+        return withOptionalValue.timeout(on: scheduler, seconds: seconds, substituteValue: nil)
+    }
+
+    func timeout(
+        on scheduler: Scheduler,
+        seconds: TimeInterval,
+        substituteValue: Value
+    ) -> Guarantee<Value> {
+        let substitute: Guarantee<Value> = Guarantee<Void>
+            .after(on: scheduler, seconds: seconds)
+            .map(on: scheduler) { substituteValue }
+        return Guarantee<Value>.race(on: scheduler, [self, substitute])
+    }
+}