implement aiter

the caller is expected to make sure that the passed in object does in
fact supports the protocol

relates #3609

vm/src/stdlib/builtins.rs

@@ -426,6 +426,11 @@ mod builtins {
         }
     }
 
+    #[pyfunction]
+    fn aiter(iter_target: PyObjectRef, vm: &VirtualMachine) -> PyResult {
+        vm.call_special_method(iter_target, "__aiter__", ())
+    }
+
     #[pyfunction]
     fn len(obj: PyObjectRef, vm: &VirtualMachine) -> PyResult<usize> {
         obj.length(vm)