Fix Monoid / SemiGroup instances for GHC 8.4

[guibou]

Build tested with GHC 8.0.2, 8.2.2 and 8.4.3.

[conal]

Thanks. With GHC 8.0.2, I got some warnings for missing mappend definitions, so I defined them to be equal to (<>). Expected? Is there not a default mappend = (<>)?

[guibou]

https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid is saying

#if !(MIN_VERSION_base(4,11,0))
  -- this is redundant starting with base-4.11 / GHC 8.4
  -- if you want to avoid CPP, you can define `mappend = (<>)` unconditionally
  mappend = (<>)
#endif

So, apparently:

• GHC 8.0: Semigroup and Monoid are not related. To implement Monoid you need to provide mappend and mempty. Adding mappend = (<>) is correct. There is no default.
• GHC 8.4: Semigroup is a base class of Monoid: To implement Monoid you need to provide only mempty. <> comes from Semigroup. However Monoid keeps mappend which is implemented by default with <>. Implementing it is correct.

So as far as I know, if you want to keep support for GHC 8.0, you need mappend = (<>). So you are correct.

Sorry I missed that in my PR. And I apologie I missed your comment and took one month to answer.

[conal]

Thanks very much for the explanations. I'm just catching up after a very distracted week. Released to Hackage.