[Pitch] `Span` over a single value

Hello all, this is a new proposal to add initializers to form a single-element Span over any value, and to form a single-element MutableSpan over any mutable value. The evolution pull request is at [pitch] `Span` over a single value by glessard · Pull Request #3445 · swiftlang/swift-evolution · GitHub, and most of the content follows. Motivation Occasionally, programmers need adaptors between single values and Span-taking API. Currently it is possible to use the span property of CollectionOfOne ...