Draft: core: Print stacktrace to help locating unassigned variable streams

A lot easier than going through the trouble of debugging where they happen.

Just not sure if we should tabulate these somehow, to only print this stack with sources only once, plus times it happened, and interrupt simulation when Simulator::Run is called. Thinking of a straightforward way to check these things for reproducibility.

So we would need to collect these first, then check if it is not assigned later. But in this meantime, because the global stream was already incremented, everything else is shifted regardless.