Development of deck API. * Separate `Deal` class from `Deck`, roughly analogous to `Pool` vs. `Die`. * `Deal` can now output multiple hands. The counts for each hand are provided in order to `eval.next_state`. * `comb_row` now uses iterative rather than recursive memoization. This will prevent some stack overflows. * `Pool`s are not permitted to be constructed using a raw `Die` or `Deck` argument. * `reduce` argument of `Die.equals()` renamed to `reduce_weights`.