xo-interpreter2: scaffold repl + alloc measurement frameowkr

include/xo/facet/box.hpp

@@ -38,6 +38,7 @@ namespace xo {
             {}
 
             /** (copy ctor not supported -- ownership is unique) **/
+            box(const box & other) = delete;
 
             // --------------------------------
 
@@ -73,9 +74,11 @@ namespace xo {
 
             ~box() {
                 auto p = this->data();
+                if (p) {
                     this->_drop();
                     ::operator delete(p);
                 }
+            }
         };
     } /*namespace facet*/