Support lazy enumerators for @stream

[rmosolgo]

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Fixes #4907

TODO:

• [ ] Maintain compatibility with existing batching
• [ ] Add a test for this behavior in particular
• [ ] Find an implementation other than recursion

[rmosolgo]

Looking at how this played out, I have a few big questions still to work out:

• Currently, each list item calls the next one via recursion. This will make really big Ruby stacks, hogging memory. A better approach would be "trampolining", something I spiked several years ago, where different steps of execution actually return to the main caller, which dispatches steps. That change might be necessary to support this feature effectively.
• The current batching behavior expects each list item to request its own data, then resolve requests for all list items in a single batch. That's impossible with this approach where each item is handled separately. How can I support both without expensive .is_a? checks on every object? Maybe detect @stream in particular?

[rmosolgo]

This is still very much on my radar, but I don't plan to continue work on this branch in particular.