* Remove a bunch of Default(Bidirectional|RandomAccess)Indices usage from stdlib and test
* Remove some DefaultRandomAccessIndices and IndexDistance usage from Foundation
* Remove no-longer-used internal type in Existentials.swift
* Get rid of indicesForTraversal
* Eradicate IndexDistance associated type, replacing with Int everywhere
* Consistently use Int for ExistentialCollection’s IndexDistance type.
* Fix test for IndexDistance removal
* Remove a handful of no-longer-needed explicit types
* Add compatibility shims for non-Int index distances
* Test compatibility shim
* Move IndexDistance typealias into the Collection protocol