highjson-0.4.0.0 build failure with GHC 8.4

Add semigroup instances
See also: https://ghc.haskell.org/trac/ghc/wiki/Migration/8.4#SemigroupMonoidsuperclasses

```
[2 of 2] Compiling Data.HighJson ( src/Data/HighJson.hs, .stack-work/dist/x86_64-osx/Cabal-2.2.0.1/build/Data/HighJson.o )

/Users/dan/scratch/highjson-0.4.0.0/src/Data/HighJson.hs:49:10: error:
• Could not deduce (Semigroup (a :& b))
arising from the superclasses of an instance declaration
from the context: (Monoid a, Monoid b)
bound by the instance declaration at src/Data/HighJson.hs:49:10-48
• In the instance declaration for ‘Monoid (a :& b)’
|
49 | instance (Monoid a, Monoid b) => Monoid (a :& b) where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

I was able to reproduce this locally like so:

```bash
stack unpack highjson-0.4.0.0 && cd highjson-0.4.0.0
echo 'resolver: nightly-2018-05-10' > stack.yaml
stack build --test --bench --no-run-benchmarks
```

Start with src/Data/HighJson.hs, especially the Monoid instance at line 49 and the GHC 8.4 error shown in the issue. Reproduce the failure with the supplied stack unpack and build commands using resolver nightly-2018-05-10. Done means highjson-0.4.0.0 builds successfully with the test and benchmark flags under GHC 8.4.

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