CharacterView is now entirely redundant in Swift 4. Deprecate its
use. This also allows us to schedule the unbreaking of
String.CharacterView leakiness without a hard source break.
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.
