When stream closes, also close the iterator

Readable (Duplex) responds to iterator return() and throw() by closing/throwing the stream, but it does not communicate stream disconnects back to the iterator.

It would be good for the Readable to call return() and throw() on the iterator on stream disconnects.

Start by finding the code that converts iterable sources into Readable/Duplex Node.js streams, then read the tests for iterator return() and throw() behavior. Add coverage for stream disconnects propagating back to the iterator. Done means closing or erroring the stream calls the iterator's return() or throw() as appropriate without breaking existing stream behavior.

Written by the indexing model from the issue text.