Semiring Maybe instance is badly behaved

[hdgarrood]

Semiring has the following law:

• zero * a = zero

With the instance (Semiring a) => Semiring (Maybe a), neither of these hold for a = Nothing - since mul is defined as liftA2 mul, zero * Nothing = Nothing, but zero = Just zero. Changing the definition of zero to be Nothing could fix this but might break other laws, I'm not sure. Maybe we should just remove this instance.