subordinate states `completed` and `canceled` under `resolved`

- This better mirrors the state structure of the underlying `Future`
revert addition of an argument to the `emit`ting of `willContinue` and `didContinue` events

- Observers already have access to the object they’re observing
- `didContinue` observers can just inspect the pipeline’s `args`
(`values`, if the api/property name changes)
- `willContinue` observers can preview the upcoming iteration by
inspecting the pipeline’s `iterator.yielded.value`
add type check for function value in error state before applying it
expound/proof/reorg
add markdown headings