Split GameObjectCollision into two classes

[GregHib]

Having an instance dedicated to add and remove would remove the branching used and could come with some performance gains.

Also probably some other things to improve like

private fun modifyCardinal(x: Int, y: Int, level: Int, block: Int, add: Boolean) {
        val flags = collisions.flags[Zone.tileIndex(x, y, level)] ?: return
        val index = Tile.index(x, y)
        flags[index] = flags[index] or CollisionFlags.blocked[1 or block] or CollisionFlags.blocked[3 or block] or CollisionFlags.blocked[5 or block] or CollisionFlags.blocked[7 or block]
}

The 1,3, 5, 7 could be combined into a single array if block is replaced with some kind of ordinal value