refactor focusing on xo-alloc2/ xo-gc/ write-barrier

ability to inform allocator of gco->gco mutation, via AAllocator i/face.

src/procedure2/ObjectPrimitives.cpp

@@ -133,7 +133,7 @@ namespace xo {
             assert(!cell->is_empty());
 
             if (!cell->is_empty()) {
-                cell->assign_head(rcx.collector(), dest);
+                cell->assign_head(rcx.allocator(), dest);
             }
 
             return cell;