[mod] rename stack to queue

[dariogoetz]

This addresses #103 and renames the occurring stacks to queue.

Two things that might require discussion:

1. The name of the method unstack does not make sense anymore. The corresponding queue version pop is too weak in my opinion. I now named it resolve_queued_event, which is rather long. Do you have a good idea for a name?
2. There are some occurrences where a closure of a now Queued object is called s (coming from the name Stacked). Shall these be renamed? To q for Queued? Or e for a generic "event"?