ExtensionArray isin
Some (many?) extension array *scalar* types will have a corresponding *container* type. For example,  `IPArray` defines `isin` to match the standard library's behavior where an address can be in a network.

https://github.com/ContinuumIO/cyberpandas/blob/054d3a37fe84a8d222795c31bbf99ef34be90fbf/cyberpandas/ip_array.py#L349-L388

Do we want to dispatch `isin` to Extension arrays? https://github.com/pandas-dev/pandas/pull/20522 is doing some work so that `pd.core.algos(categorical)` dispatches to a specialized (faster) `.isin`, so the additional work will be small.