Proposal: Add operator to separate out signals for inner errors and inner completions

:warning: Not Ready to Merge :warning:

I'd really like to see a way to capture error and completion events from inner signals. I've had various ideas go through my head, but I figured it was time to take a stab at something, and get feedback. Is there's a better way to this? This could go a bunch of different directions.

No, the name isn't serious, `-trifurcate:` is just me punting on the decision of what to call it.