feat update

src/exec/just_test.cc

@@ -0,0 +1,9 @@
+#include "sled/ref_count.h"
+#include <gtest/gtest.h>
+#include <sled/exec/just.h>
+
+TEST(just, basic)
+{
+    sled::just_sender<int> sender = sled::just(1);
+    auto op = connect(sender, sled::cout_receiver{});
+}