GC refactoring: make evacuate() take an StgClosure**

GC refactoring: make evacuate() take an StgClosure**

Change the type of evacuate() from
  StgClosure *evacuate(StgClosure *);
to
  void evacuate(StgClosure **);

So evacuate() itself writes the source pointer, rather than the
caller.  This is slightly cleaner, and avoids a few memory writes:
sometimes evacuate() doesn't move the object, and in these cases the
source pointer doesn't need to be written.  It doesn't have a
measurable impact on performance, though.