[kotlin] Traverse workflow tree once per render pass instead of twice

[zach-klippenstein]

Currently we do two tree traversals per render pass:

1. Render: Generate renderings, create new child workflows/workers, tear down old ones.
2. Tick: Recursively select on all workers and event channels.

We should be able to reduce this to a single pass:

1. Wrap the render pass in a select, and pass the SelectBuilder down through all the render calls.
2. WorkflowNode.runningWorker adds each worker to the SelectBuilder as soon as it's ran.
3. WorkflowNode.renderWithStateType adds its event channel to the SelectBuilder before returning.
4. Once the root render call returns, offer the rendering to the rendering channel (it's conflated so no need to send).
5. When the select returns, send the output to the output channel if non-null.

[zach-klippenstein]

Nevermind, this isn't a great idea:

1. The 2 passes is a constant factor, doesn't affect time complexity.
2. Don't actually have a reason to think this is noticeably affecting performance for anyone.
3. Merging passes will make the code a lot messier.
4. Once WIP Optimize rendering by caching renderings when possible. #803 lands, the first pass won't be a full traversal anyway.