Rewrite the fn #getPotentialKingAttacks;
previously, b/c in actual games, opponent kings are never on adjacent squares,
this fn returned an empty array; however, from the perspective of the
game service, this assumption is flawed.
Refactor the fns #getPotentialMoves and
#getPotentialKingAttacks with the helper fn #get-potential-moves.
Redo spec for k.getPotentialKingAttacks.
Redo spec for gs.vulnerable-positions-for.