NativeCPP: Do GC callbacks better for Boehm
NativeCPP: Do GC callbacks better for Boehm

We now actually use the collector's event callback to trigger the
Rogue callback, so it should fire whenever the GC runs, not just
when it's run by hand.

AbePralle/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue,AbePralle/Rogue