Support the new family of `Span` types

[MahdiBM]

Please summarize your feature request

Support the new family of Span types

Describe the functionality you're seeking in more detail

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.

We should support Span as an input, in public API.

Have you considered any alternatives?

No response

[MahdiBM]

At a glance MultipartPartBodyElement and generally ArraySlice<UInt8> usages seem like good candidates for replacement by Span<UInt8>.
We probably can't get rid of all these if we have to support Swift 6.0 and 6.1 for now.