Add exercises for Monoid (and maybe Semigroup)

[gwils]

Monoid is perhaps the most core and widespread type class which is currently missing from the course. A module should be added on it.

Perhaps we should also add a module on Semigroup, in which case we should add NonEmptyList to differentiate them. If we do this, we should change the some exercise in the alternative module to return NonEmpty

Discussion point: Should we first introduce Semigroup then Monoid, following the hierarchy? I think it might be more enlightening to teach Monoid first - the more key abstraction - and then move on to Semigroup to answer the question "what happens without mempty".

[puffnfresh]

I think you could argue either direction but I'm probably on the side of Semigroup first, for the same reason as Functor first, not Monad.

[endgame]

HPFFP does Monoid first, but I'm not especially fussed.