Adds a convenience for mapping Never outputs to other types

[jszumski]

Adds a way to map a workflow's Never output type to a different type without generating compiler warnings. Fixes #339

[CLAassistant]

All committers have signed the CLA.

[jamieQ]

thanks! left some minor comments. also a test that this new method 'does the right thing' should be added, probably to AnyWorkflowTests.swift

[jszumski]

Great feedback, I also added a simple test for it

[square-tomb]

This reminds me of ReactiveSwift's promoteError. I notice theirs takes the target type as an explicit parameter:

let (numbersSignal, numbersObserver) = Signal<Int, Never>.pipe()
let (lettersSignal, lettersObserver) = Signal<String, NSError>.pipe()

numbersSignal
    .promoteError(NSError.self)
    .combineLatest(with: lettersSignal)

I wonder if there are situations where that parameter is necessary for type inference to be succeed or be performant?

[jamieQ]

lgtm – thanks!