message: elle.print: Handle `std::weak_ptr`.
diff: @@ -155,6 +155,13 @@ namespace elle
  o << "nullptr";
  }
  + template <typename T>
  + void
  + print(std::ostream& o, std::weak_ptr<T> const& value)
  + {
  + print(o, value.lock());
  + }
  +
  inline
  void
  print(std::ostream& o, void* value)
  