[AltStoreCore] Refactors core AltStore logic into AltStoreCore framework

AltStoreCore will contain all shared AltStore code between AltStore and any app extensions. Initially, it includes all AltStore model logic.

AltStoreCore/Extensions/JSONDecoder+Properties.swift

@@ -0,0 +1,64 @@
+//
+//  JSONDecoder+Properties.swift
+//  Harmony
+//
+//  Created by Riley Testut on 10/3/18.
+//  Copyright © 2018 Riley Testut. All rights reserved.
+//
+
+import Foundation
+import CoreData
+
+public extension CodingUserInfoKey
+{
+    static let managedObjectContext = CodingUserInfoKey(rawValue: "managedObjectContext")!
+    static let sourceURL = CodingUserInfoKey(rawValue: "sourceURL")!
+}
+
+public final class JSONDecoder: Foundation.JSONDecoder
+{
+    @DecoderItem(key: .managedObjectContext)
+    public var managedObjectContext: NSManagedObjectContext?
+    
+    @DecoderItem(key: .sourceURL)
+    public var sourceURL: URL?
+}
+
+public extension Decoder
+{
+    var managedObjectContext: NSManagedObjectContext? { self.userInfo[.managedObjectContext] as? NSManagedObjectContext }
+    var sourceURL: URL? { self.userInfo[.sourceURL] as? URL }
+}
+
+@propertyWrapper
+public struct DecoderItem<Value>
+{
+    public let key: CodingUserInfoKey
+    
+    public var wrappedValue: Value? {
+        get { fatalError("only works on instance properties of classes") }
+        set { fatalError("only works on instance properties of classes") }
+    }
+    
+    public init(key: CodingUserInfoKey)
+    {
+        self.key = key
+    }
+    
+    public static subscript<OuterSelf: JSONDecoder>(
+        _enclosingInstance decoder: OuterSelf,
+        wrapped wrappedKeyPath: ReferenceWritableKeyPath<OuterSelf, Value?>,
+        storage storageKeyPath: ReferenceWritableKeyPath<OuterSelf, Self>
+    ) -> Value? {
+        get {
+            let wrapper = decoder[keyPath: storageKeyPath]
+
+            let value = decoder.userInfo[wrapper.key] as? Value
+            return value
+        }
+        set {
+            let wrapper = decoder[keyPath: storageKeyPath]
+            decoder.userInfo[wrapper.key] = newValue
+        }
+    }
+}