Make `Defaults` be an enum instead of a class

Sources/Defaults/Defaults.swift

@@ -1,7 +1,7 @@
 // MIT License © Sindre Sorhus
 import Foundation
 
-public final class Defaults {
+public enum Defaults {
 	public class Keys {
 		public typealias Key = Defaults.Key
 
@@ -79,8 +79,6 @@ public final class Defaults {
 		}
 	}
 
-	fileprivate init() {}
-
 	/// Access a defaults value using a `Defaults.Key`.
 	public static subscript<Value: Codable>(key: Key<Value>) -> Value {
 		get { key.suite[key] }

Sources/Defaults/Observation.swift

@@ -65,7 +65,7 @@ extension Defaults {
 		return try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(dataValue) as? Value
 	}
 
-	final class BaseChange {
+	struct BaseChange {
 		let kind: NSKeyValueChange
 		let indexes: IndexSet?
 		let isPrior: Bool