yet another repair for backtraces
yet another repair for backtraces

Compacting of the old generation breaks backtrace info. We
could try to fixup backtrace info, but it's simpler to just
disable compaction.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket