how to wait for async operations inside iterator of: Stream.reduce

When consuming a stream with [`.reduce`](https://caolan.github.io/highland/#reduce), I'd like to be able to do some async operations inside the `iterator/reducer` function and have the stream wait for their completion before consuming next element.

Here's what I'm doing right now:

```js
const H = require('highland')
const Promise = require('aigle')

const reducer = async (memoPromise, x) => {
console.log('got x:', x)
const memo = await memoPromise
console.log('processing x:', x)
return Promise.delay(10).then(() => memo + x)
}

H([1, 2, 3])
.reduce(reducer, 0)
.toPromise(Promise)
.then(console.log)
```

However the stream is being consumed as fast as possible, without waiting for async operation inside the reducer to complete. The output of running the above is:
```
got x: 1
got x: 2
got x: 3
processing x: 1
processing x: 2
processing x: 3
6
```

while I'd like it to be:
```
got x: 1
processing x: 1
got x: 2
processing x: 2
got x: 3
processing x: 3
6
```

Start with the Stream.reduce implementation and documentation, then reproduce the provided reducer example to observe when each input is consumed. Define done as reducer processing occurring sequentially, with the final result still resolving to 6 and the output matching the requested order.

Written by the indexing model from the issue text.