CLN: Single way to get underlying values for Index / Series
(split from https://github.com/pandas-dev/pandas/pull/19520)

It'd be convenient to have an internal method for getting the highest-fidelity array stored by a container (Index, Series, Block?).

For `Series`, this is already what `._values` does. `Index._values` is sometimes different though (Period, DatetimeIndex with TZ), so we'll use a different name.

dtype           | array type
--------------- | ----------
category        | Categorical
datetime64ns    | ndarray
datetime64ns-tz | DatetimeIndex
interval        | IntervalIndex (eventually IntervalArray)
numeric         | ndarray
period          | PeriodIndex (eventually PeriodArray)
sparse          | SparseArray
str             | ndarray

