Monoid now has Semigroup as superclass -> Monoid chapter needs reworking

[schmittlauch]

Since base-4.18, Monoids are required to be Semigroups as well. This breaks all Monoid examples in the book.

We should probably introduce the Semigroup typeclass before monoids and move all mappend implementations to the (<>) function of Semigroup.

First of all: Are you interested in making this the repo where the book is kept up to date? Then I may do the changes later once I feel safe with using Haskell.

Have you ever contacted the author about keeping the book up to date or getting his sources?