[racket] Callback on shutdown

Would it make sense for will executors [^1] to have a definition of
"unreachability" that includes GC, as now, but also immanent program
termination (and they run automatically on termination)?

Will executors have a per-"object" granularity. That seems like how
you'd usually prefer to express it -- and best for organizing and
maintaining code, and using third-party modules that cooperate
smoothly?


[^1]: http://docs.racket-lang.org/reference/willexecutor.html