Add TODO comment to iomux impl

src/iomux_impl.h

@@ -43,6 +43,8 @@ struct IOMux {
     /* Used by mux implementations to hold their own data/instance */
     void *instance;
 };
+//TODO - have iomux_xx_init embed struct IOMux and get rid of IOMux.instance
+//for better cache friendiness
 
 int iomux_select_init(struct IOMux *mux);
 int iomux_epoll_init(struct IOMux *mux);