Non-frozen enums in Swift

[Karl]

Extensible Enumerations for Non-Resilient Libraries

Hey folks. Over the weekend I put together a quick draft patch that would enable non-resilient Swift libraries to provide "extensible" enums, where adding a new case would not be a Semver major breaking change. This patch does the absolute minimum required to enable the functionality, so before I go any further with it I wanted to get the community's opinion on whether this is a problem worth addressing. Below is a draft proposal that covers the issue. Please give it a read and let me know your…