Constant mmtk::policy::immix::NEVER_MOVE_OBJECTS

pub const NEVER_MOVE_OBJECTS: bool = _; // true

In some cases/settings, Immix may never move objects. Currently we only have two cases where we move objects: 1. defrag, 2. nursery copy. If we do neither, we will not move objects. If we have other reasons to move objects, we need to add them here.