Supporting Span in packages

[FlorianPircher]

@glessard

The Span family of types includes: Span<T>, RawSpan, MutableSpan<T>, MutableRawSpan, OutputSpan<T> and OutputRawSpan. Note that UTF8Span does not have back-deployment support.

OutputSpan appears as appleOS 26+, so not backdeployed as far as I can tell. There are no docs for OutputRawSpan, so difficult to tell its deployment.

Using `Span` on pre-26 Apple OS versions

Array.span, String.utf8.span and Substring.utf8.span aren't back-deployable due to the bridging changes they depend on.

How can I use backdeployed span types? I cannot figure out how to create a value of these span types targeting macOS 15 and earlier.