awaitingStreamMoveNext property

Whether the pending suspension is an await for's moveNext().

SCE16. This used to be awaitingStreamConversion, and the name was accurate: the loop suspended ONCE on stream.toList() and then walked the list. That made await for eager — every element was produced before the body ran once, a break could not stop the producer, and a loop over an infinite stream never ran its body at all because toList() never completes. The loop now suspends once per element on StreamIterator.moveNext, which is what await for means.