[Pitch] Add `span` properties to Optional

Hello all, this is a new proposal to add span and mutableSpan properties to Optional, along with an edit(_:) function similar to UniqueArray.edit(_:). The swift-evolution pull request is at [pitch] Add span properties to Optional by glessard · Pull Request #3489 · swiftlang/swift-evolution · GitHub, and most of its contents follows. Summary of changes Adds span and mutableSpan computed properties to Optional. They provide in-place access to the wrapped value as a one-element Span or MutableSpan,...