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.

src/save.c

@@ -1109,6 +1109,7 @@ freedynamicdata(void)
     freeroleoptvals(); /* saveoptvals(&tnhfp) */
     cmdq_clear(CQ_CANNED);
     cmdq_clear(CQ_REPEAT);
+    cmdbind_freeall();
     free_tutorial(); /* (only needed if quitting while in tutorial) */
 
     /* per-turn data, but might get added to when freeing other stuff */