`SQUARES_180` is a misleading name

[jacksonthall22]

I always assumed SQUARES_180 was mapping e.g. a1 -> h8, b1 -> g8, etc, until I actually needed to use it, and realized it's just a vertical mirror, a1 -> a8, b1 -> b8. It's used directly in `square_mirror() - maybe deserves a different name?