Rework player package to use structures and a hash table

Instead of an association list and a type contract, let's use a hash
table and a structure.  This is: more explicit, encapsulates state
more effectively, generates much of the cruft (constructors,
destructors) for me.

A hash table is more efficient when considering large player bases.
In particular, we'll be generating unique player IDs to make it a bit
more difficult for a malicious agent to guess another player's ID and
make bad requests.

packages.lisp

@@ -58,13 +58,12 @@
    :5d.model)
   (:export
    :player
-   :players
    :error-player-nonexistent
    :error-player-broke
-   :player-id :player-balance :player-hand
-   :player-exists? :player-bankrupt? :player-can-bet?
-   :player-pay
-   :player-receive))
+   :player-exists? :player-bankrupt? :player-can-pay?
+   :player-debit
+   :player-credit
+   :player-set-cards))
 
 (defpackage cantedraw.game
   (:nicknames :5d.game)