[Pitch] Add `indexed()` and `Collection` conformances for `enumerated()` and `zip(_:_:)`

[Jumhyn (Frederick Kellison-Linn)]

This is thanks to @Slava_Pestov

Availability checking for protocol conformances

I just finished landing a set of changes which fix a hole in Swift's availability checking model. While this was mostly intended for use by Apple, I feel that it is also worth documenting and communicating to the community since people do ship third-party frameworks that make use of OS availability. As most of you are aware, Swift's "availability" feature checks if referenced declarations are going to be available at runtime by comparing the OS version in an @available annotation with the "avai…