Icepool

* This will probably be the last version with a `VectorDie` distinction.
* Dice cannot have negative weights.
* `VectorDie` cannot be nested inside tuple outcomes.

* Die and dict arguments to `Die()` are now expanded, including when nested.
* Add `Die.if_else()` method, which acts as a ternary conditional operator on outcomes.
* Dice are now hashable. `==` and `!=` return dice with truth values based on whether the two dice have identical outcomes and weights.
* `ndim` now uses singletons `icepool.Scalar` and `icepool.Empty`.

* `EvalPool.eval()` can now be provided with single rolls of a pool.
This can be a dict-like mapping individual die outcomes to counts or a sequence of individual die outcomes.
* `EvalPool.next_state()` can not expect that the outcomes it sees are consecutive,
though they are guaranteed to be seen in monotonic order.
* `FindBestRun()` no longer assumes consecutive outcomes.
* Added `DicePool.sample()`.
* Added `die.truncate()`.
* `min_outcomes`, `max_outcomes` (for pool definitions) renamed to `truncate_min`, `truncate_max`.
* Removed `die.getitem()`.
* `DicePool` is no longer iterable, since there isn't an intuitive, unambiguous way of doing so.
* `align_range()` now only operates on scalar outcomes.

* Renamed from `hdroller` to `icepool`.
* Primary repository is now https://github.com/HighDiceRoller/icepool.
* Only `tuple`s become `VectorDie`.
* Weights of 10^30 or above are not shown in tables by default.

* Add `max_depth` parameter to `sub()`. If set to `None` this seeks a fixed point.
* Add `'reduce'` option for `denominator_method` parameters.
* Cache results of `repeat_and_sum()`.

* Add `ndim` keyword argument to `d()`.
* Removed `hitting_time()` method; it seems too niche to commit to.
* Several arguments are now keyword-only.
* Fix to `reroll_until()` for vector dice.