Icepool

* Pool scorers can now return `None` states, which drops the state from evaluation, effectively performing a full reroll.
* Operations that result in no remaining weight (e.g. rerolling all outcomes) now return None.
* Dice can no longer be constructed with zero total weight.
* The `Die()` constructor now auto-trims.
* `Die._align()` is now private and no longer makes all the dice the same weight.
* Fixed `Die.explode()` weighting.
* Cleaned up unused functions.

* Optimize `Pool.pops()` by immediately removing all the dice if there are no left in the mask.
* `Die.reroll` now tracks weights properly.

* Implemented new keep/pool algorithms, which make heavy use of caching. The `keep*()` function signatures changed.
* `Die` is now hashable.
* Empty `Die` is now an error.
* Removed weight normalization.
* Added `%` and `//` operators to `Die`.
* Renamed `ccdf` to `sf` following `scipy`'s naming.

* `best_set` now queries the `score_func` to determine possible outcomes, which allows more flexibility.

* `Die.align()` and `Die.trim()` are now public.
* Optimize `keep()` algorithm by skipping to results after passing the last kept die.
* Add `best_set()` method for computing sets of matching dice.

The major change in this version is a new roll-and-keep algorithm. This can keep arbitrary indexes, as well as supporting dissimilar dice as long as they share a common "prefix" of weights.