Fix warnings for Xcode 12, when used from SPM

Dwifft/AbstractDiffCalculator.swift

@@ -7,7 +7,9 @@
 //
 
 import Foundation
-
+#if canImport(UIKit)
+import UIKit
+#endif
 /// A parent class for all diff calculators. Don't use it directly.
 public class AbstractDiffCalculator<Section: Equatable, Value: Equatable> {
 
@@ -57,7 +59,7 @@ public class AbstractDiffCalculator<Section: Equatable, Value: Equatable> {
         #if os(iOS) || os(tvOS)
             let row = indexPath.row
         #endif
-        #if os(macOS)
+        #if os(macOS)  || os(watchOS)
             let row = indexPath.item
         #endif
         return self.sectionedValues[indexPath.section].1[row]

Dwifft/Dwifft.swift

@@ -36,10 +36,10 @@ public enum DiffStep<Value> : CustomDebugStringConvertible {
     /// The value to be inserted or deleted.
     public var value: Value {
         switch(self) {
-        case let .insert(j):
-            return j.1
-        case let .delete(j):
-            return j.1
+        case let .insert(_, value):
+            return value
+        case let .delete(_, value):
+            return value
         }
     }
 }

Package.swift

@@ -1,13 +1,11 @@
-// swift-tools-version:5.1
+// swift-tools-version:5.2
 //
 
 import PackageDescription
 
 let package = Package(
     name: "Dwifft",
     platforms: [
-        .iOS(.v8),
-        .tvOS(.v9),
         .macOS(.v10_11)
     ],
     products: [