BUG/API: index.array for RangeIndex, MultiIndex
ATM MultiIndex.array raises ValueError, since there is no single array actually backing MI.  RangeIndex creates and returns a PandasArray.  I think RangeIndex should also raise (at least until/unless we implement a RangeArray).

This also has implications for extract_array, which currently raises on MultiIndex, which has led to some ugly checks in e.g. Index._cmp_method.