Implement some simple converters for custom types to/from integers

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.

packages.lisp

@@ -34,7 +34,13 @@
 
 (defpackage cantedraw.model
   (:use :cl :cantedraw.lib.macros :cantedraw.lib.functions)
-  (:export :int-card :rank :suit :card :cardset))
+  (:export
+   ;; Types
+   :int-card :rank :suit :card :cardset
+   ;; Converters
+   :int->suit :int->rank :int->card
+   :suit->int :rank->int :card->int
+   ))
 
 (defpackage cantedraw.main
   (:use :cl :cantedraw.lib.macros :cantedraw.lib.functions)