Missing "join" method on ADTs

Based on my understanding of many of the ADTs, as described [here](https://mostly-adequate.gitbooks.io/mostly-adequate-guide/ch09.html#mixing-metaphors), I think crocks is missing `join`.

Here is a simplified use case (I realize I could create this function with more of the functional building-blocks Crocks supplies):

```js
// { } -> String -> Either(Error String, a)
const getData = payload => {
return field => {
const data = payload[field]

// hmmm..
if (!data) {
return Left(new Error(`Cannot get field "${data}"`))
} else {
return Right(data)
}
}
}

const result = Either(
getData({ 10: 100, 'dogs': 'cats' })
)
.ap(Right('dogs'))

console.log(result)
// Right Right 'cats'
```

The problem is that the result is a `Right` inside a `Right`.

I'd like to be able to do this:

```js
const result = Either(
getData({ 10: 100, 'dogs': 'cats' })
)
.ap(Right('dogs'))
.join()

console.log(result)
// Right 'cats'
```

Am I missing something? Is there a different way to use `ap` that wouldn't create nested types?