Ensure custom types support pickling.

[ragnard]

Ensure NamedRowTuple pickle roundtrips successfully.

Added tests for all custom types.

Fixes #490.

Description

Implement necessary pickle methods to ensure type can be pickled/unpickled correctly.

Non-technical explanation

Support pickle serialization/deserialization for rows.

Release notes

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* Ensure custom types support pickling. ({issue}`490`)