Event Emitters and Streams

[nrfulton]

UIs built around LLM interactions tend to co-mingle streams at two different levels of abstraction:

1. The stream coming out of the llm itself llm_stream : str -> stream[...]
2. Higher-level events: things like tool calls, iterations on a retry loop, requirement checks (which passed and failed), etc.

We should have some sort of interface for emitting both token/str streams and also events.