Generically's Monoid instance uses wrapped type's (<>)

[yairchu]

PR for #17

Note that Generically now does not have a Monoid instance unless it wraps something with a Semigroup instance, so had to add an instance derivation in the test too.

[Lysxia]

Ah yes, DerivingVia should be hidden behind a CPP block for older GHCs. (Maybe it would be a good idea in the future to split tests relying on post-8.6 in separate files...)

[yairchu]

Oh! So instead of that I've made the instance with a bit of boiler-plate so it will also work pre 8.6.

[Lysxia]

Thanks!