Use swift result instead of custom result

[niil-ohlin]

What has been done

• Remove custom Result enum.
• Reimplement Result as a typealias to Swift.Result constrained on Error
• Deprecate Result.getValue() in favor of the provided Result.get().
• Remove Result.init(_:) since it is a duplication of Result.init(catching:). (This will have a minimal API change because most of the time trailing closure syntax will have been used, but it's still breaking nonetheless.)
• Remove map and flatMap since it is a pure reimplementation of the standard library functions.

[niil-ohlin]

I tried running our app with this change and only had to change one line:

-   bag += signal.map(Flow.Result.success).onValue(completion)
+   bag += signal.map { Flow.Result.success($0) }.onValue(completion)

because the type checker could not resolve the type. I think that this warrants a major release.