Add stateless workflow convenience extensions to Swift

[zach-klippenstein]

In Kotlin, we recently (#219, #213) refactored our Workflow type to be a general type, with two subtypes: StatefulWorkflow (was Workflow) and StatelessWorkflow (something we had before, but now more obviously a directly first-class citizen and sibling of StatefulWorkflow). So far the feedback from Android devs has been positive.

This is not a huge change, mostly just naming, so should we adopt it in Swift as well?

cc @davidapgar @timdonnelly @bencochran

[rjrjr]

Alternative for Swift:

When a Workflow’s state is Void, it’s typically meaningless to have a makeInitialState or workflowDidChange that’s different than the following:

// Helper for stateless workflows. (Use the consistent terminology.)
extension Workflow where State == Void {
    func makeInitialState() -> State {
        return ()
    }
    func workflowDidChange(from previousWorkflow: Self, state: inout State) {
    }
}

We should consider providing a default extension on Workflow when State == Void that implements those as such. If a workflow needs to, say, perform a side effect in either of those, they can still implement them and not use the default implementation.

[rjrjr]

Seems reasonable to diverge here, since Swift is nicer than Kotlin can be.