Implement stepped slicing for tuples

tests/snippets/3.1.3.2.py

@@ -8,3 +8,14 @@ assert squares == squares[:]
 assert [1, 9, 25] == squares[::2]
 assert [4, 16] == squares[1::2]
 assert [4] == squares[1:2:2]
+
+squares_tuple = (1, 4, 9, 16, 25)
+
+assert 1 == squares_tuple[0]
+assert 25 == squares_tuple[-1]
+assert (9, 16, 25) == squares_tuple[-3:]
+assert squares_tuple == squares_tuple[:]
+
+assert (1, 9, 25) == squares_tuple[::2]
+assert (4, 16) == squares_tuple[1::2]
+assert (4,) == squares_tuple[1:2:2]

vm/src/objlist.rs

@@ -9,7 +9,7 @@ fn get_pos(l: &Vec<PyObjectRef>, p: i32) -> usize {
     }
 }
 
-fn get_slice_items(l: &Vec<PyObjectRef>, slice: &PyObjectRef) -> Vec<PyObjectRef> {
+pub fn get_slice_items(l: &Vec<PyObjectRef>, slice: &PyObjectRef) -> Vec<PyObjectRef> {
     // TODO: we could potentially avoid this copy and use slice
     match &(slice.borrow()).kind {
         PyObjectKind::Slice { start, stop, step } => {

vm/src/objtuple.rs

@@ -20,29 +20,16 @@ pub fn get_item(vm: &mut VirtualMachine, l: &Vec<PyObjectRef>, b: PyObjectRef) -
                 Err(vm.new_exception("Index out of bounds!".to_string()))
             }
         }
-        PyObjectKind::Slice { start, stop, step } => {
-            let start = match start {
-                &Some(start) => get_pos(l, start),
-                &None => 0,
-            };
-            let stop = match stop {
-                &Some(stop) => get_pos(l, stop),
-                &None => l.len() as usize,
-            };
-            let step = match step {
-                //Some(step) => step as usize,
-                &None => 1 as usize,
-                _ => unimplemented!("stepped slicing not supported for type {:?}", l),
-            };
-            // TODO: we could potentially avoid this copy and use slice
-            let obj = PyObject::new(
-                PyObjectKind::Tuple {
-                    elements: l[start..stop].to_vec(),
-                },
-                vm.get_type(),
-            );
-            Ok(obj)
-        }
+        PyObjectKind::Slice {
+            start: _,
+            stop: _,
+            step: _,
+        } => Ok(PyObject::new(
+            PyObjectKind::Tuple {
+                elements: super::objlist::get_slice_items(l, &b),
+            },
+            vm.get_type(),
+        )),
         _ => Err(vm.new_exception(format!(
             "TypeError: indexing type {:?} with index {:?} is not supported (yet?)",
             l, b