[Pitch] UTF8Span: Safe UTF-8 Processing Over Contiguous Bytes

[benrimmington]

@xwu

@Michael_Ilseman

We will add utf8Span-style properties to String and Substring, in line with however SE-0456 turns out.

This is a good caveat. To make it explicit, if we buy the argument that storage-is-better-than-span for the property, this would be utf8Storage; and if we buy the argument that span should be the Span-vending API, then this would be utf8Span?

Alternatively, var contiguousUTF8: UTF8Span? to match the existing isContiguousUTF8 and makeContiguousUTF8() APIs. Should the result be optional, or would it have the same "eager copy behaviour" as SE-0456?