API:  MultiIndex.labels -> codes
The boat may have long sailed on this, but just for consideration.

I semi-frequently get the `.levels` and `.labels` of a `MultiIndex` backwards.  Maybe it's just me, but I think `labels` is the culprit, because in other pandas contexts, "labels" refer to the actual value of the thing.  E.g.
- `.loc` indexes by "labels" (values)
-  the values inside of a single row index or columns are the "labels" for those items

So, consistent with `Categorical`, would it make sense for the integer mapping inside a `MultiIndex` to also be called `.codes`?
