stream: support passing generator functions into pipeline

Add support for generators and functions in pipeline.

This makes it possible to do the following:

await pipeline(
  async function* () {
    yield await read();
  },
  async function*(source) {
    await for (const chunk of source) {
      yield chunk;
    }
  },
  async function(source) {
    await for (const chunk of source) {
      await write(chunk):
    }
  }
);

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines