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/hack.h

@@ -252,7 +252,7 @@ struct cmd {
     boolean swap_yz;       /* QWERTZ keyboards; use z to move NW, y to zap */
     const char *dirchars;      /* current movement/direction characters */
     const char *alphadirchars; /* same as dirchars if !numpad */
-    const struct ext_func_tab *commands[256]; /* indexed by input character */
+    struct Cmd_bind *cmdbinds;
     const struct ext_func_tab *mousebtn[NUM_MOUSE_BUTTONS];
     char spkeys[NUM_NHKF];
     char extcmd_char;      /* key that starts an extended command ('#') */