convert Subscription from bare operation to iterator.

[cowboyd]

Motivation

fixes #693

In order to simplify working with streams and subscriptions, we need to be able to disambiguate them easily. The relationship between Stream and Subscription is the same as that between Iterable -> Iterator and AsyncIterable -> AsyncIterator.

Approach

This converts Subscription into an API that is highly analogous to AsyncIterator. The next() method returns an Operation which yields an IteratorResult So in the same way AsyncIterator has:

next(): Promise<IteratorResult>;

The Subscription api will look like:

next(): Operation<IteratorResult>;

Once we have this in place, it is trivial to create a stream out of any Subscription:

function streamable(subscription: Subscription): Stream {
  return ({
    *[Symbol.iterator]() {
      return subscription;
    }
  });
}

We may decide later to rename Stream and Subscription to something else that re-enforces this analogy further. However, we haven't been able to settle on anything that obviously clicks, so we'll defer that for now, and instead focus on the API.