* 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
Collection's popFirst is only present when the Collection is its own
SubSequence type. String and String's views are no longer their own
SubSequenes, so popFirst is no longer present. Unfortunately, this
breaks code in swift-version 3 and it also gives a terrible diagnostic
to users. This change introduces an implementation for swift-version 3
mode and better diagnostics for Swift 4 code. Tests included.
Since samePosition(in:) now unconditionally returns optionals,
String.UTF16View.Index.distance(to: String.UTF16View.Index) must accept an
optional to keep some code working.
Fixes <rdar://33307780>.
It is still possible to get the same behavior by providing an explicit
type context, or in a generic code, but otherwise, `String("")!` will
not compile.
