Declare attackers always combines attacker + defender choosing

Declaring attackers is not split between choosing attackers (508.1a) first, then by announcing which defender(s) (508.1b)

> If the defending player controls a planeswalker, the person who cast Master Warcraft first chooses the complete group of creatures that are going to attack. Then, for each of those creatures, the active player chooses who or what it’s going to attack.