runtime: implement newcoro, coroswitch to support package iter

testdata/go1.23/main.go

@@ -1,14 +1,29 @@
 package main
 
+import "iter"
+
 func main() {
 	testFuncRange(counter)
+	testIterPull(counter)
+	println("go1.23 has lift-off!")
 }
 
-func testFuncRange(f func(yield func(int) bool)) {
-	for i := range f {
+func testFuncRange(it iter.Seq[int]) {
+	for i := range it {
+		println(i)
+	}
+}
+
+func testIterPull(it iter.Seq[int]) {
+	next, stop := iter.Pull(it)
+	defer stop()
+	for {
+		i, ok := next()
+		if !ok {
+			break
+		}
 		println(i)
 	}
-	println("go1.23 has lift-off!")
 }
 
 func counter(yield func(int) bool) {

testdata/go1.23/out.txt

@@ -8,4 +8,14 @@
 3
 2
 1
+10
+9
+8
+7
+6
+5
+4
+3
+2
+1
 go1.23 has lift-off!