Serialization / Deserialization of ExtensionArrays
(just getting an issue number to link to. Will update later)

What hooks do we want to provide for ExtensionArrays stored inside our containers?

Currently, to_csv works. Haven't really tried others

- pickle
- msgpack
- parquet
- HDF5
- json

xref https://github.com/pandas-dev/pandas/pull/20611/files