Fix IndexPath slicing crash

Author: PatrickPijnappel

Sources/Foundation/IndexPath.swift

@@ -261,9 +261,9 @@ public struct IndexPath : ReferenceConvertible, Equatable, Hashable, MutableColl
                     case 0:
                         return .empty
                     case 1:
-                        return .single(slice[0])
+                        return .single(slice.first!)
                     case 2:
-                        return .pair(slice[0], slice[1])
+                        return .pair(slice.first!, slice.last!)
                     default:
                         return .array(Array<Int>(slice))
                     }