Joker constructor takes a rank for uniqueness - rank doesn't really
change anything in terms of the "power" of a joker but helps with
ensuring jokers are unique in a deck.
Deck constructor takes an optional argument for the number of decks.
A deck includes two jokers, so n decks include 2n jokers.
Given n in 0..51, certainly there exists r, s s.t. `n = 13s + r`
where r in 0..12 and s in 0..3. `r` is the rank, and `s` is the suit.
ranks are ordered Ace, 2, 3, ..., Queen, King and suits are ordered
Diamond, Club, Heart, Spade.
Therefore there is a 1-1 correspondence between 0..51 and any card in
the deck.
Jokers are a bit less straightforward; for now I'll define them as an
outlier which doesn't exist in the same bounds (which is why they're
the default return for suit in int->suit). Likely I'll go for -1
representing a Joker.
