Icepool

* 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()`.

* `mean()`, `variance()`, etc. now return an exact `fractions.Fraction` when possible. (Note that `Fraction`s only support float-style formatting from Python 3.12.)
* Rename `disjoint_union` to `additive_union`.
* Add `keep_negative_counts` keyword argument to `+, -, &, |` binary operators for multiset expressions (default `False`).
* Symmetric difference (`^`) for multiset expressions is now a straight absolute difference of counts.
* Add unary `+` operator for multiset expressions, which is the same as `keep_counts(0)`.

Improve some error messages.

Fix a bug in `MultisetExpression.keep_outcomes()` and `drop_outcomes()` regarding unbinding variables.