API: support multiple indexers for .iloc with a MultiIndex
MultIndexing with multiple indexers (#6301) via `.loc` is great.

It would be nice to mirror this functionality with `.iloc`.

To my understanding, until this change, `loc` and `iloc` had a mirror syntax, where if you replaced all of your index labels with arrays of 0-indexed integers, they were equivalent, e.g., for the following series:

```
import pandas as pd
midx = pd.MultiIndex.from_product([range(3), range(5)])
s = pd.Series(range(15), midx)
```

Now they lack this symmetry, because indexing like `s.iloc[0, 0]` doesn't work like `s.loc[0, 0]`. I found this surprising. Thoughts?
