Merge pull request swiftlang#21845 from numist/numist/diffing

Ordered Collection Diffing

Author: Catfish-Man

stdlib/public/core/CMakeLists.txt

@@ -200,7 +200,9 @@ set(SWIFTLIB_SOURCES
   ${SWIFTLIB_ESSENTIAL}
   ### PLEASE KEEP THIS LIST IN ALPHABETICAL ORDER ###
   Availability.swift
+  CollectionDifference.swift
   CollectionOfOne.swift
+  Diffing.swift
   ExistentialCollection.swift.gyb
   Mirror.swift
   PlaygroundDisplay.swift