[DGD] callout cantrip

[Shentino]

Ok, so I was sitting back for a bit, experimenting with callouts, and I
suddenly had the bright idea of trying to pass a callout its callout handle
as an argument.

Naturally, owing to causality, not very straightforward.

Apparently arguments on pending callouts share the object's dataspace, so
what I was able to do was pass a one element array, have the callout
scheduled, and then modify the array in place ala TLS/call_trace, thus
giving the callout information on what its handle was.