Decide whether (new) ExtensionArrays and Dtypes are public
This mostly affects 

1. IntervalArray
2. PeriodArray
3. DatetimeArray (and maybe TimedeltaArray if we do that)
4. SparseArray
5. IntegerArray

Categorical is already public, so let's leave that out.

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A few questions

1. do we allow users to construct these directly (via a set of `to_*_array` methods, or a top-ish-level `pd.array([...], dtype)` method)?
2. Do users see these when they do `.values` or any operation returning an array (`.unique`, probably others)?