Introduce inspect_core operator

[frankmcsherry]

Generalizes inspect to present both data events and frontier advancement events. Fixes TimelyDataflow/differential-dataflow#338

[antiguru]

I like the idea! Otoh, I'm not too fond of using Result to disambiguate frontiers and data. It seems like stretching the concept. As an alternative, why not provide two functions, or use a specialized enumeration?

[danhhz]

I like the idea! Otoh, I'm not too fond of using Result to disambiguate frontiers and data. It seems like stretching the concept. As an alternative, why not provide two functions, or use a specialized enumeration?

https://docs.rs/timely/0.12.0/timely/dataflow/operators/to_stream/enum.Event.html is even already pretty close to the specialized enum you'd need