Stores a list of reviews for a card, in chronological order. Each FSRSItem corresponds
to a single review, but contains the previous reviews of the card as well, after the
first one.
When used during review, the last item should include the correct delta_t, but
the provided rating is ignored as all four ratings are returned by .next_states()
Function type for post scheduling operations that takes interval, maximum interval,
current day index, due counts per day, and a random number generator,
and returns a new interval.