Icepool

* Rerolls in pools, `mix()`, and `sub()` are now triggered by a sentinel value `hdroller.Reroll` (rather than `None`).
* Single-`int` indexes for `count_dice` and pool `[]` operator now produce a die rather than a pool,
in analogy to sequence indexing. However, note that this is absolute relative to the whole pool
rather than relative to any previous indexing.
* `EvalPool` can now iterate in both directions even if one of `max_outcomes` or `min_outcomes` is given,
though at lower efficiency in the disfavored direction.
* Default such direction changed back to ascending.
* Re-implemented `clip()`.
* The minimum outcome of `highest()` is equal to the highest minimum outcome among all input dice
and likewise for `lowest()`. This will prevent introducing zero-weight outcomes where they did not exist before.

* Pool `max_outcomes` or `min_outcomes` outside the range of the fundamental die is now an error.
* Pools no longer automatically shrink the die to fit `max_outcomes` or `min_outcomes`. This is to guarantee a predictable iteration order.
* Improved `str(die)` formatting.
* `VectorDie` outcomes are automatically converted to tuples.
* `ScalarDie` now has `ndim='scalar'` rather than `ndim=False`.
* Add `standard_pool()` function for easy creation of standard pools.
* Removed `reroll_lt()`, etc. as they weren't enough faster than `reroll(func)` to justify spending extra space.

* Removed `initial_state()` in favor of using `None` as the initial state.
* Added simple `pool.eval()` method.
* `None` now rerolls in `EvalPool`.
* Fix `trunc`, `floor`, `ceil`.
* Various improvements to pool algorithm.
* Add some thresholding methods for dice.
* Max/min renamed to highest/lowest.
* Improve empty die handling.
* Improved `VectorDie` string formatting.
* Disabled `reverse()`.

* Faster algorithm for keeping the single highest/lowest.
* Fix `from_sweights()`.

* Fix `max()` and `min()` casting to dice.

* Capitalize `Die()` and `Pool()` factory methods.
* `PoolEval` renamed to `EvalPool`.
* Implemented `min_outcomes` for pools. Direction is now determined by `EvalPool`.
* Allow dice with zero weights or zero outcomes.
* `BaseDie` subclasses are now `ScalarDie` and `VectorDie`.
* `==` and `!=` compare outcomes again. Dice are no longer hashable by the standard `hash()`.
* Rewored `Pool()` arguments.
* Added several methods.