Initial import of diffing package into standard library

Author: numist

stdlib/public/core/CMakeLists.txt

@@ -202,8 +202,10 @@ set(SWIFTLIB_SOURCES
   ### PLEASE KEEP THIS LIST IN ALPHABETICAL ORDER ###
   Availability.swift
   CollectionOfOne.swift
+  Diffing.swift
   ExistentialCollection.swift.gyb
   Mirror.swift
+  OrderedCollectionDifference.swift
   PlaygroundDisplay.swift
   CommandLine.swift
   SliceBuffer.swift