Add List::len method

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/list.rs

@@ -185,6 +185,12 @@ impl List {
         FunctionResult::ReturnPush(item)
     }
 
+    pub(crate) fn len(vm: &mut Vm, _state: FunctionState) -> FunctionResult {
+        let this = vm.frame_stack()[0].clone();
+        let len = with_obj_downcast(this, |list: &List| list.list().len());
+        Int::create(len as i64).into()
+    }
+
     pub(crate) fn push(vm: &mut Vm, _state: FunctionState) -> FunctionResult {
         let this = vm.frame_stack()[0].clone();
         let arg = vm.frame_stack()[1].clone();