Add a convenience for mapping outputs from `Never` to any type that does not produce compiler warnings

[jamieQ]

today, attempting to do this both requires an instance of the new type and produces a compiler warning. we should offer a special case for this that makes the type signatures 'just work' without requiring a value and doesn't emit a compiler warning. example:

struct WF: Workflow {
    typealias Output = Never
    typealias State = Void
    typealias Rendering = Int

    func render(state: State, context: RenderContext<WF>) -> Rendering {

        return 0
    }
}

func test(_ rc: RenderContext<WF>) {
    enum NewOutput {
        case zero
    }

    let z: AnyWorkflow<String, NewOutput> = WF()
        .mapRendering { "\($0)" }
        .mapOutput { _ in .zero } // ⚠️ Will never be executed
        .asAnyWorkflow()

}