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/decl.c

@@ -247,6 +247,7 @@ static const struct instance_globals_c g_init_c = {
     /* decl.c */
     UNDEFINED_VALUES, /* chosen_windowtype */
     0, /* cmd_key */
+    NULL, /* cmd_bind */
     0L, /* command_count */
     UNDEFINED_PTR, /* current_wand */
 #ifdef DEF_PAGER