API: Standardize ExtensionDtype subtype name
We have a few extension arrays that build off one or more ndarrays. You'll often want to get the underlying NumPy dtype for that array.

* IntegerArray uses `.type`
* IntervalArray uses `.subtype`

IntervalArray can't use `.type`, since that has to be `Interval`. So IntegerArray will need to alias subtype to type.

Are we happy with the name `subtype`?
