Icepool

The data of a die resulting from `==` or `!=` is lazily evaluated.

This saves computation in case the caller is only interested in the truth value.

`EvalPool` favors the cached direction more.

Major reworking of pool construction.

* Public constructor is now just `Pool(*dice)`.
* In particular, no more `truncate_min` or `truncate_max` arguments.
* Pools can be of arbitrary dice, though non-truncative sets of dice will have lower performance. There is some performance penalty overall.
* `apply()` called with no arguments now calls `func` once with no arguments.

* Removed `Die.keep()`. Use `Die.pool(...).sum()`.
* `highest`/`lowest` returns empty die if any of the input dice are empty.

* Free-function form of `lowest`, `highest` now selects between algorithms for better performance and generality.
* Removed `die.lowest, `die.highest`.
* `die.min_outcome`, `die.max_outcome` no longer take arguments beyond `self`.
* No more (public) `repeat_and_sum`, as this is redundant with `` and `die.keep()`.
* No more public `die.keep_lowest_single()`, `die.keep_highest_single()`.
* `count_dice` arguments are now keyword-only.
* `die.zero()` no longer reduces weights to 1.
* Update PyPi classifiers.

* Removed `min_outcome` parameter from die construction.
* Operations on tuple outcomes are now performed recursively, to match the fact that die expansion on construction is recursive.
* Renamed `Die.reduce()` to `Die.reduce_weights()`.
* Added `reduce()` and `accumulate()` functions analogous to the functions of the same name from functools/itertools.
* Add CSV output.
* Renamed `Die.markdown()` to `Die.format_markdown()`.
* Added `star` parameter to `sub`, `explode`, `reroll`, `reroll_until` methods. If set, this unpacks outcomes before giving them to the supplied function.
* Added experimental `JointEval` class for performing two evals on the same roll of a pool.