Change MultiIndex repr (labels -> codes)
As I mentioned in https://github.com/pandas-dev/pandas/pull/22511#issuecomment-451667687, the repr for MultiIndex ATM uses ``labels``, even though the parameter/attribute has been changed to ``codes``.

```python
>>> mi = pd.MultiIndex.from_product([[8,9,0], ['a', 'b', 'c']])
>>> MultiIndex(levels=[[0, 8, 9], ['a', 'b', 'c']],
...            labels=[[1, 1, 1, 2, 2, 2, 0, 0, 0], [0, 1, 2, 0, 1, 2, 0, 1, 2]])
```

This PR changes the repr, so it uses ``codes``.

Fot the record I think we should either decide to adapt #22511 soon, or soon come up with an consensus about an alternative repr to be implemented. The current MultiIndex repr is really, really (really!) bad for large indexes and needs to be be changed.
