Async/await-based effects

[gordonbrander]

Instead of using Combine, we can take advantage of Swift's new concurrency primitives.

Fixes #26

[gordonbrander]

One tradeoff with this approach is that we do want combine-like publisher-subscription semantics for things like subconsciousnetwork/subconscious#473 (comment). If we moved forward with this approach, we should still have an answer for how to support subscribing to combine publishers.

[gordonbrander]

An alternative approach might be to continue to use combine publishers as our Fx mechanism, but create an extension that bridges async and Future.

extension Future where Failure == Never {
    convenience init(_ run: @escaping () async -> Output) {
        self.init { promise in
            Task {
                 let result = await run()
                 promise(.success(result))
            }
        }
    }
}

To limit the use of long-running effects, we could change the Fx type to Future<Action, Never>, and make the default constructor for Update take a future, while holding an AnyPublisher for batched effects.

[gordonbrander]

Closed in favor of #32