Icepool

* `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.

* `MultisetExpression.map_counts()` now accepts multiple arguments.
* `MultisetExpression.keep_outcomes()` and `drop_outcomes()` now accept an expression as an argument.
* `MultisetExpression.highest_outcome_and_count()` now returns the min outcome if no outcomes have positive count.

* `highest`, `lowest`, and `middle` can now take a single iterable argument.
* Add `all_straights` evaluation.
* `all_counts` and `all_straights` now output sizes in descending order.
* Harmonize method and free function versions of `map`. Both now allow extra arguments and repeat.
* Rename `filter_counts` to `keep_counts`.
* `expand` evaluator now allows order to be set.
* Experimental `compair` evaluation.

* Add HTML and BBCode options for population formatting.
* Renamed `apply` to `map` and the decorator version to `map_function`.
* The above now uses `guess_star`.
* Add default of 1 die for `Die.pool()`.