Add the DoubleWidth type

Co-authored-by: Nate Cook <natecook@apple.com>
Co-authored-by: Max Moiseev <moiseev@users.noreply.github.com>
Co-authored-by: Xiaodi Wu <13952+xwu@users.noreply.github.com>

Author: 4 people

Package.swift

@@ -3,7 +3,7 @@
 //
 // This source file is part of the Swift Numerics open source project
 //
-// Copyright (c) 2019 Apple Inc. and the Swift Numerics project authors
+// Copyright (c) 2019-2021 Apple Inc. and the Swift Numerics project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -71,7 +71,7 @@ let package = Package(
 
     .testTarget(
       name: "IntegerUtilitiesTests",
-      dependencies: ["IntegerUtilities"],
+      dependencies: ["IntegerUtilities", "_TestSupport"],
       exclude: ["CMakeLists.txt"]
     ),
 

Sources/_TestSupport/CMakeLists.txt

@@ -1,13 +1,14 @@
 #[[
 This source file is part of the Swift Numerics open source project
 
-Copyright (c) 2019-2020 Apple Inc. and the Swift Numerics project authors
+Copyright (c) 2019-2021 Apple Inc. and the Swift Numerics project authors
 Licensed under Apache License v2.0 with Runtime Library Exception
 
 See https://swift.org/LICENSE.txt for license information
 #]]
 
 add_library(_TestSupport
+  DoubleWidth.swift
   Error.swift
   Interval.swift
   RealTestSupport.swift)