Like notetypes, there is a col.get_deck() routine which caches
fetches, so that successive fetches are cheap. This makes it simpler
to just fetch the deck at the start.
We were also attempting to fetch a deck with id 0 for each row; I've
changed this so that we only fetch it if the id is non-zero.
I18n uses an Arc internally, so it is cheap to clone. This allow us
to drop the lifetime specifiers on the context structures.