message: Add getter for self pointer
diff: @@ -74,6 +74,10 @@ public:
  then_impl(f, e);
  }
  + Self* self() {
  + return self_;
  + }
  +
  private:
  template <class F>
  void await_impl(F& f) const {
  @@ -172,6 +176,9 @@ public:
  self_->varargs_receive(rc, mid_, std::move(ef), std::move(ca));
  }
  + Self* self() {
  + return self_;
  + }
  private:
  message_id mid_;
  