libutil: add box_ptr<T>::take

sometimes it's useful to turn a box into a unique_ptr. let's have that.

Change-Id: I3d142c6e157c5e61c16f3953b3b8b4cd616f0e93

lix/libutil/box_ptr.hh

@@ -72,6 +72,11 @@ public:
     }
 
     box_ptr(box_ptr<T> & other) = delete;
+
+    std::unique_ptr<T> take() &&
+    {
+        return std::move(inner);
+    }
 };
 
 template<typename T>