rust-index: return variables systematic naming convention

To help knowing at a glance when a method is ready, making
us more comofortable when we are close to the final removal of
scaffolding, we introduce the systematic variable names `rust_res` and
`c_res`. The goal of this series is to always return the formet.

We take again the case of `pack_header` as example.

Our personal opinion is to usually avoid such poor semantics as `res`, but
usually accept it when it close to the actual return, which will be the
case in most methods of this series. Also, the name can simply be dropped
when we remove the scaffolding. To follow on the example, the body of
`pack_header()` should become this in the final version:

```
    let index = self.index(py).borrow();
    let packed = index.pack_header(args.get_item(py, 0).extract(py)?);
    Ok(PyBytes::new(py, &packed).into_object());
```

in these cases it is close to the actual return and will be removed
at the end entirely.

rust/hg-cpython/src/revlog.rs

@@ -218,10 +218,11 b' py_class!(pub class MixedIndex |py| {'
     def pack_header(&self, *args, **kw) -> PyResult<PyObject> {
         let rindex = self.index(py).borrow();
         let packed = rindex.pack_header(args.get_item(py, 0).extract(py)?);
-        let packed = PyBytes::new(py, &packed).into_object();
-        let cpacked = self.call_cindex(py, "pack_header", args, kw)?;
-        assert_py_eq(py, "pack_header", &packed, &cpacked)?;
-        Ok(packed)
+        let rust_res = PyBytes::new(py, &packed).into_object();
+
+        let c_res = self.call_cindex(py, "pack_header", args, kw)?;
+        assert_py_eq(py, "pack_header", &rust_res, &c_res)?;
+        Ok(rust_res)
     }
 
     /// get an index entry