Add an option to build the concurrency library for back deployment

Implemented by Doug Gregor, a member of the Swift team at Apple, this unexpected pull request adds an option to build the new Swift Concurrency libraries for older versions of macOS/iOS. These libraries can then be embedded in targets deployed before macOS 12/iOS 15 to support the new concurrency features. I'm still not sure if this makes the new concurrency features fully available when working with older OS versions, but this is something to watch.

Open Sourcing Identified Collections

Most applications involve working with collections of data in some form, and it's common to model such data as a standard Array. However, as applications grow more complex, the limitations of Arrays might start to cause problems. Thankfully, this open-source library, created by the Point Free team, can help. It provides a new type called IdentifiedArray for working with collections of identifiable elements in a performant way.