refactor coroutine lowering and tasks

src/internal/task/task_none.go

@@ -0,0 +1,29 @@
+// +build scheduler.none
+
+package task
+
+import "unsafe"
+
+//go:linkname runtimePanic runtime.runtimePanic
+func runtimePanic(str string)
+
+func Pause() {
+	runtimePanic("scheduler is disabled")
+}
+
+func Current() *Task {
+	runtimePanic("scheduler is disabled")
+	return nil
+}
+
+//go:noinline
+func start(fn uintptr, args unsafe.Pointer) {
+	// The compiler will error if this is reachable.
+	runtimePanic("scheduler is disabled")
+}
+
+type state struct{}
+
+func (t *Task) Resume() {
+	runtimePanic("scheduler is disabled")
+}