Change key binds from array to linked list

Key bindings were stored as a fixed-size array, indexed by the input
character, pointing to the extended commands.  This changes that into
a linked list of an intermediary struct Cmd_bind, storing the input key
and the pointer to the command.

This is just code cleanup for future enhancements, and should have
no effect on gameplay.

include/decl.h

@@ -240,6 +240,7 @@ struct instance_globals_c {
     /* decl.c */
     char chosen_windowtype[WINTYPELEN];
     int cmd_key; /* parse() / rhack() */
+    struct Cmd_bind *cmd_bind;
     cmdcount_nht command_count;
     /* some objects need special handling during destruction or placement */
     struct obj *current_wand;  /* wand currently zapped/applied */