[Pitch] `UncheckedString` (raw string support for Swift)

Hello all. This is a new proposal to add an UncheckedString<Element> type that can be used to hold strings that are not necessarily valid UTF-8, including strings whose encoding isn't known, and strings with wide characters. The evolution pull request is at [Proposal] UncheckedString (raw string support for Swift) by al45tair · Pull Request #3488 · swiftlang/swift-evolution · GitHub. Please see the current version for more (it's too long to fit in a post :-))