Fix mixed sync/async callback results typings

[wiktor-obrebski]

Fix bug in types when async and sync results has been mixed in map callback, e.g.

  const asyncFun = () => {
    return Promise.resolve(123 as const);
  };

  const mapped: Result<123, Error1 | Error2> = Result.combine(mixedResults)
    .map(() => {
      if (Math.random()) {
        return Err.of(new Error2());
      }

      return asyncFun();
    })

This gave a typescript error before this change.