xo-alloc2: ICollector_Xfer + ICollector_DX1Collector [WIP]

include/xo/alloc2/arena/DArena.hpp

@@ -64,6 +64,8 @@ namespace xo {
 
             ///@}
 
+            size_type allocated() const { return free_ - lo_; }
+
             /** obtain uncommitted contiguous memory range comprising
              *  a whole multiple of @p hugepage_z bytes, of at least size @p req_z,
              *  aligned on a @p hugepage_z boundary

include/xo/alloc2/arena/IAllocator_DArena.hpp

@@ -8,9 +8,7 @@
 #include "arena/DArena.hpp"
 
 namespace xo {
-    namespace mm {
-        struct IAllocator_DArena;
-    }
+    namespace mm { struct IAllocator_DArena; }
 
     namespace facet {
         template <>