[stdlib] Move DoubleWidth into its own file

Author: natecook1000

stdlib/public/core/CMakeLists.txt

@@ -47,6 +47,7 @@ set(SWIFTLIB_ESSENTIAL
   CString.swift
   CTypes.swift
   DebuggerSupport.swift
+  DoubleWidth.swift.gyb
   DropWhile.swift.gyb
   Dump.swift
   EmptyCollection.swift