cl: compile async functions

x/async/_demo/gendemo/gendemo.go

@@ -0,0 +1,21 @@
+package main
+
+import "github.com/goplus/llgo/x/async"
+
+func GenInts() (co *async.Promise[int]) {
+	print("1")
+	co.Yield(1)
+	print("2")
+	co.Yield(2)
+	print("3")
+	co.Yield(3)
+	print("4")
+	return
+}
+
+func main() {
+	co := GenInts()
+	for !co.Done() {
+		print(co.Next())
+	}
+}