[AltStoreCore] Backports iOS 15+ NSManagedObjectContext.performAndWait<T>()

Simplifies returning values and throwing errors from managed object contexts.

AltStoreCore/Extensions/NSManagedObjectContext+Conveniences.swift

@@ -0,0 +1,37 @@
+//
+//  NSManagedObjectContext+Conveniences.swift
+//  AltStore
+//
+//  Created by Riley Testut on 5/16/23.
+//  Copyright © 2023 Riley Testut. All rights reserved.
+//
+
+import CoreData
+
+public extension NSManagedObjectContext
+{
+    // Non-throwing
+    func performAndWait<T>(_ closure: @escaping () -> T) -> T
+    {
+        var result: T!
+        
+        self.performAndWait {
+            result = closure()
+        }
+        
+        return result
+    }
+    
+    // Throwing
+    func performAndWait<T>(_ closure: @escaping () throws -> T) throws -> T
+    {
+        var result: Result<T, Error>!
+        
+        self.performAndWait {
+            result = Result { try closure() }
+        }
+        
+        let value = try result.get()
+        return value
+    }
+}