Add exercises for Contravariant/Divisible/Decidable?

[endgame]

While we wouldn't get to them in the 3-day version of the course, they are useful but still pretty much folklore. A number of libraries use them (waargonaut, sv, hasql, discrimination, ...?) but in terms of learning material, there is an Ed talk and a GeorgeW talk but not much else.

Having a bunch of "write the instances" exercises with a blog post announcing them would help fill this gap.

[puffnfresh]

I think this would be super useful. I try to solidify the relationship between Functor, Applicative, Monad and I think Contravariant would especially help.

[endgame]

We'll likely need modules on Semigroup and Monoid first, which means we'll need NonEmpty to differentiate them.

[gwils]

I am in favour of all these things.