Use cattrs hooks for (un)structuring

[wpbonelli]

Currently we hand-write some field-specific unstructuring logic inside one big unstructure_component hook. This is convenient because we also reshape period data in the same place. But eventually it would be cleaner to have cattrs (un)structuring hooks for every type we might see in incoming data, except for containers (dicts/tuples) which cattrs will handle automatically.