Icepool

Testing GitHub workflows.

Expanded multiset processing with multiset expressions.

* `OutcomeCountGenerator`, `OutcomeCountEvaluator` renamed to `MultisetGenerator`, `MultisetEvaluator`.
* `multiset_function` is an easy way to create joint evaluators.
* `Pool` indexing is now relative rather than absolute.
* Renamed `pool.sorted_roll_counts` to `pool.keep` and `pool.keep_tuple`.
* `Die` versions of `sum_highest` etc. renamed to just `highest`; these always return dice.
* `MultisetExpression` (including generators like `Pool`) have `highest()` returning an expression.
* `Die` operators now does mixed vector-scalar binary operations by broadcasting the scalar.
* Add `middle()` methods.
* Remove `*generators` argument from `evaluator.order()`, `evaluator.final_outcome()`.
* `*generators` argument of `evaluator.alignment` replaced with the union of all generator outcomes.
* Removed suits.
* Stop using `__class_getitem__`, which is intended for typing only.

Reworked built-in generators and evaluators.

* Generators now support multiset operations. These include:
* Multiset comparisons (`<, <=, >, >=, ==, !=, isdisjoint`), which produce `Die[bool]`.
* Multiset operators (`+`, `-`, `|`, `&`, `^`) which produce wrapped generators.
* A suite of chainable count adjustments, including `multiply_counts`, `divide_counts`, `filter_counts`, and `unique`.
* Rename `median_left` to `median_low` etc.
* Generators and evaluators are now paramterized by count type as well.
* Move concrete evaluators to a submodule.

* Fixed weighting bug in `__matmul__` when the left die has an outcome of 0.
* Retired the names `standard` and `bernoulli`. These will be just `d` and `coin` respectively.
* `apply` now accepts `Pool` arguments, which will have `expand()` called on them.
* Reinstate automatic Cartesian product in `Population` construction.
* `if_else` now runs in two stages.

* Incremental sorting for `all_matching_sets` to reduce state space.
* `lowest()` and `highest()` now actually visible.
* Improved checking for tuple outcome sortability and types.

Prepend `sum_` to OutcomeCountGenerator versions of `highest` and `lowest`.