Retired the `EmptyDie` / `ScalarDie` / `VectorDie` distinction. * There is now only one `Die` class; it behaves similarly to how `ScalarDie` used to. * There is no more `ndim`. * The `[]` operator now forwards to the outcome, acting similar to what `VectorDie.dim[]` used to do. * Removed `PoolEval.bind_dice()`. It was cute, but I'm not convinced it was worth spending API on.