Derive Semigroup instances for GHC8.4+

Hello, we are currently using postgresql-query in a project that we are updating to stackage LTS 12.0 which uses GHC 8.4.3. Unfortunately, postgresql-query does not currently build against GHC 8.4.3 due to the Semigroup as a Superclass of Monoid proposal here.

Fortunately, it looks like the solution is quite simple: derive instances of Semigroup for the types that already have instances of Monoid in Database/PostgreSQL/Query/Types.hs. I will try to submit a PR once I figure out how to work bitbucket (I'm a github user) unless you would like to just make the change yourself.

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