[rust] Clarify whether StreamResult::Dropped should have a count

### Background

I wrote this version of pread the other day:
```rust
async fn pread(fd: &Descriptor, size: usize, offset: u64) -> Result, ErrorCode> {
let (mut rx, future) = fd.read_via_stream(offset);
let data = Vec::::with_capacity(size);
let mut bytes_read = 0;
let (mut result, mut data) = rx.read(data).await;
loop {
match result {
StreamResult::Complete(n) => {
assert!(n <= size - bytes_read);
bytes_read += n;
assert_eq!(data.len(), bytes_read);
if bytes_read == size {
break;
}
(result, data) = rx.read(data).await;
}
StreamResult::Dropped => {
assert_eq!(data.len(), bytes_read);
break;
}
StreamResult::Cancelled => {
panic!("who cancelled the stream?");
}
}
};
drop(rx);
match future.await {
Ok(()) => Ok(data),
Err(err) => Err(err),
}
}
```
Based on the signature of `StreamResult::Dropped`, I assumed that no bytes would have been transferred. But @alexcrichton kindly pointed out that in fact, the assertion here is actually incorrect:

```rust
StreamResult::Dropped => {
assert_eq!(data.len(), bytes_read);
break;
}
```

That is to say, `Dropped` can also be accompanied by data, and you can detect that via `data.len()` being larger than it was before the previous `write` / `write_buf`.

### The issue

I would like for `Dropped` to have an `items-transferred` argument, like `Complete`. Strictly speaking it duplicates what is happening to `data.len()`, so another consistent option would be to remove `items-transferred` from `Complete`.

WDYT?