Vm refactorings

Here are some refactorings to make the vm code simpler. For example I removed the `nursery_collector` class because it's just an alias for `copying_collector<aging_space, nursery_policy>`. And the bump_allocator template class was only ever instantiated with object, so then it is easier if it is a real class. Things like that.
ooh, its empty definition.  thats kinda funny.