* feat(map): Add the ability to map over elements of a collection inside a reactor.
* improvement: throw a more helpful error when a step returns an invalid result.
* fix: automatically pass extra arguments from the map step to nested steps.
They can't be referred to directly because they may be added to the graph
at runtime, and may depend on steps which have been completed and their
results discarded because they have no dependents at that time.
* fix: spurious test failures seemingly caused by `Mimic`.
