Icepool

* Rename `keep_counts` to `keep_counts_ge`. Add `le`, `lt`, `gt`, `eq`, and `ne` variants.
* Add `count_subset` evaluation that counts how many times the right side is contained in the left.
* Add `ImplicitConversionError` as subclass of `TypeError`.
* Add binary multiset operators to `Deck`.
* Add `modulo_counts` / `%` operation on multisets.
* Rebind generators and evaluate when fully bound non-generator expressions are given to an evaluator.
* Fix `Symbols` intersection.
* Fix argument order in `__rfloordiv__`.

* Fix `Symbols` operator priority with `Population`, `AgainExpression`.
* Added experimental `map_to_pool` and `explode_to_pool` methods.
* Split `compair` into `compare_lt` etc.
* Constructing a mixture of dice now effectively uses the old `lcm_joint` method, which reduces the denominator more aggressively.

Improve some error messages.

* 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()`.

If this goes well, it may become v1.0.0.