Drop an old todo item.

I spend some time trying out alternatives to `void *` as the item type,
but the alternatives did not seem better.

In particular:
* the `intptr_t` type is fundamentally considered an int type, so that
  storying pointers in it still requires a cast; and
* unions sometimes have weird casting requirements as well.

My current opinion is that a simple `void *` is still the best option
that I'm aware of.