Icepool

* Providing only a `drop` argument to `lowest()` or `highest()` will now keep all other elements rather than just the first non-dropped element.
* `depth` argument to `Die.reroll()` is now mandatory.
* `tuple` outcomes are now auto-`tupleize`d again during `Die` construction.
* Add `Die.stochastic_round()` method.
* Add `percent` option to `Population.probability` methods.
* Add new `again_count` mode for handling `Again`, which limits the total number of dice.
* Improved ability to `keep` from both ends for certain types of multiset expressions.
* Rename `func` parameters to `function`.
* Improved sorting for `Symbols`; now compares counts in alphabetical order.
* Experimental `Die.reroll_to_pool()` method.
* Experimental `all_straights_reduce_counts` and `argsort` multiset evaluations.

* Rename `keep_counts` to `keep_counts_ge`. Add `le`, `lt`, `gt`, `eq`, and `ne` variants.
* Add `count_subset` evaluation that counts how many times the right side is contained in the left.
* Add `ImplicitConversionError` as subclass of `TypeError`.
* Add binary multiset operators to `Deck`.
* Add `modulo_counts` / `%` operation on multisets.
* Rebind generators and evaluate when fully bound non-generator expressions are given to an evaluator.
* Fix `Symbols` intersection.
* Fix argument order in `__rfloordiv__`.

* Fix `Symbols` operator priority with `Population`, `AgainExpression`.
* Added experimental `map_to_pool` and `explode_to_pool` methods.
* Split `compair` into `compare_lt` etc.
* Constructing a mixture of dice now effectively uses the old `lcm_joint` method, which reduces the denominator more aggressively.

* Experimental `Symbols` class representing a multiset of characters.
* `marginals` now forwards `__getattr__` to outcomes, as long as the attribute name doesn't begin with an underscore.
* Operators on expressions now keep negative counts by default. The `keep_negative_counts` argument is retired.
* Add unary `-` for `MultisetExpression`.
* `MultisetExpression.isdisjoint()` now raises an error for negative counts.
* Small performance optimization for `Vector`.
* `Population.marginals` is no longer a `Sequence`, since the mixins don't make sense.
* `Mapping`s are now properly excluded from `Population.common_outcome_length`.
* Fixed quoting in `repr` for populations.

* Add `z(n)`, which produces a die that runs from 0 to `n - 1` inclusive.
* Add `Population.to_one_hot()`, which converts the die or deck to a one-hot representation.
* Add `Die.mean_time_to_sum()`, which computes the mean number of rolls until the cumulative sum is greater or equal to the target.

* Fix non-fully-bound case of `MultisetEvaluator.evaluate()`.
* Add `default` argument to `lowest(), highest(), middle()`.
* Add `Population.entropy()`.