Fixed a tricky type erasure bug in the scheduler

examples/y/main.orc

@@ -0,0 +1,7 @@
+import system::async::(set_timer, yield)
+import system::io::(readln, println)
+
+const main := (
+  set_timer true 1 (println "y" yield)
+    \cancel. readln \a. cancel
+)