fix broken SemigroupMonoid transition <<loop>>

Turns out that with ghc 8.2.2, the instructions given on the page don't
work. And the cppless variant that I had compiles, but into effectively
mappend = mappend so it loops.

The only way I can see to make it work without cpp is to use
mappend = (Sem.<>)
which is ugly and a land mine waiting to explode if someone changes it
to a nicer mappend = (<>) with a newer version of ghc which will compile
it and work ok, while breaking it with 8.2.2. Sigh.

I posted to haskell-cafe about this.

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