Proposal: Make Semigroup as a superclass of Monoid

[Henning Thielemann]

On Mon, 30 Mar 2015, Edward Kmett wrote:

> Collapse the first two steps into one step, but this means that nobody gets any warning of new names coming
> into the Prelude. On the other hand this would get us all the way to done by 7.14. On one hand, the first
> order consequence the amount of breakage caused by adding Semigroup to Prelude and taking (<>) is almost
> entirely confined to a handful of pretty printing libraries -- and we already broke those same libraries by
> taking (<$>).

Btw. hmatrix is another prominent package defining (<>) for matrix and 
related multiplications.