Function streamStage

streamStage(source: AsyncIterable<Program>): AsyncGenerator<Program>
Forwards one stage of a stream without letting it close the outer stream.

Drops the RESP_DONE and STREAM_END instructions, so several executors can stream into one response and only the last one finishes it.
Parameters
- source: AsyncIterable<Program>
  Chunks to forward.
Returns AsyncGenerator<Program>
The chunks, minus any terminal instructions.
Example
```
for await (const chunk of streamStage(ctx.invokeStream(draft, request))) yield chunk;
yield delta.end();
```