Implement stepped slicing for strings

vm/src/objstr.rs

@@ -33,12 +33,10 @@ pub fn subscript(vm: &mut VirtualMachine, value: &String, b: PyObjectRef) -> PyR
                 // &Some(_) => panic!("Bad stop index for string slicing"),
                 &None => value.len() as usize,
             };
-            let step2: usize = match step {
-                //Some(step) => step as usize,
-                &None => 1 as usize,
-                _ => unimplemented!(),
-            };
-            Ok(vm.new_str(value[start2..stop2].to_string()))
+            Ok(vm.new_str(match step {
+                &None | &Some(1) => value[start2..stop2].to_string(),
+                &Some(num) => value[start2..stop2].chars().step_by(num as usize).collect(),
+            }))
         }
         _ => panic!(
             "TypeError: indexing type {:?} with index {:?} is not supported (yet?)",