Multi-move turns

Supported cases of increasing complexity:
* Double move (https://en.wikipedia.org/wiki/Marseillais_chess): players just move twice each turn
* The most common variant of this has white only move once on the first move
* Progressive chess (https://en.wikipedia.org/wiki/Progressive_chess): the number of moves made per turn increases each turn
* Duck chess: players make two moves each, but the second move only moves the duck