message: Add getter for message ID
diff: @@ -72,7 +72,11 @@ public:
  then_impl(f, e);
  }
  - Self* self() {
  + message_id id() const noexcept {
  + return mid_;
  + }
  +
  + Self* self() noexcept {
  return self_;
  }
  @@ -157,7 +161,11 @@ public:
  self_->varargs_receive(rc, mid_, std::move(g), std::move(f));
  }
  - Self* self() {
  + message_id id() const noexcept {
  + return mid_;
  + }
  +
  + Self* self() noexcept {
  return self_;
  }
  