[whatwg/streams] pipeTo should return a cancelable promise (#446)

If I wanted to create:

```js
const stream = merge([stream1, stream2, stream3]);
```

Being able to pipe each stream to an identity stream would make this easy and move a lot of the work out of JS, but the missing bit is being able to react to cancellation. If `stream` is cancelled while piping `stream1`, I want to know about that so I can cancel `stream2` and `stream3`.

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