Reducer[C,M] requries Monoid[M] when Semigroup[M] would suffice

[gwils]

The Monoid[M] constraint on Reducer[C,M] is too tight. Semigroup[M] is enough.
Reducer[C,M] should be changed to only require Semigroup[M].

I assume scalaz.Reducer is based on https://hackage.haskell.org/package/monoids/docs/Data-Monoid-Reducer.html
but that has been deprecated by https://hackage.haskell.org/package/reducers/docs/Data-Semigroup-Reducer.html

[aryairani]

I think PRs get more attention than Issues do.

[S11001001]

Our Reducer couples with Generator, for which the zero is required. A semigroup-Reducer would probably require a semigroup-Generator to match.

It might be worth revisiting this in the context of Foldable/Foldable1 in general, which could use more user-controllable ways of delivering elements, but that would be a larger redesign.