Support swift concurrency

[tokijh]

Now we can use Swift Concurrency 🙌 (#1)

How to use?

@AsyncStubProxy(fetch) var fetchStub
func fetch() async -> String { 
  await fetchStub.invoke(default: "")
}

- - - - -

let convertStubStore = StubStore()
func convert<Source, Target>(_ source: Source) async -> Target {
  let stub = convertStubStore.asyncStub(arguments: Source.self, result: Target.self)
  return await stub.invokeWithoutDefault(value)
}

Unfortunately, I couldn't find a way to provide it with the same interface 😢