We're no longer currently planning on supporting caching for dynamic responses, so we can do some cleaning & simplification:
* Multiplexing can be removed since we only ever subscribe once (via `RenderResult.pipe`, described below)
* `RenderResult.toUnchunkedString` can become synchronous since static responses are never chunked
* `RenderResult.forEach` can become `RenderResult.pipe` which helps encapsulate some of the details of `RenderResult`
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