Support `Span` in `ByteBuffer`

[MahdiBM]

swiftlang/swift-evolution/proposals/0447-span-access-shared-contiguous-storage.md

The first paragraph of the introduction:

We introduce Span, an abstraction for container-agnostic access to contiguous memory. It will expand the expressivity of performant Swift code without compromising on the memory safety properties we rely on: temporal safety, spatial safety, definite initialization and type safety.

There are other Span-related proposals in the works too, which are of use: MutableSpan, OutputSpan.

[Lukasa]

Yep; we intend to do this when it is practical for non-stdlib libraries to do so.