Any previous usage can be represented in this fashion while typically
saving one function allocation and also saving the function literal’s
syntax overhead (for the seed). Plus a new feature: the provided
iterator does not have to be infinite, limiting how far expand is
allowed to go.
main work by @drewhk with contributions from @2m and @rkuhn
This work uncovered many well-hidden bugs in existing Stages, in
particular StatefulStage. These were hidden by the behavior of
OneBoundedInterpreter that normally behaves more orderly than it
guarantees in general, especially with respect to the timeliness of
delivery of upstream termination signals; the bugs were then that
internal state was not flushed when onComplete arrived “too early”.
