Add Semigroup instance for OneAnd

[liff]

Add Semigroup instance for OneAnd based on its SemigroupK.

I couldn't find any examples on how to write tests for this, so any help is appreciated.

[codecov-io]

Current coverage is 75.48%

Merging #545 into master will decrease coverage by -0.04% as of b82fa08

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  Files          153     153       
  Stmts         2063    2064     +1
  Branches        68      68       
  Methods          0       0       
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  Hit           1558    1558       
  Partial          0       0       
- Missed         505     506     +1

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[ceedubs]

👍 (assuming Travis turns green)

There's a gitter conversation going on as to whether we might support an automatic SemigroupK => Semigroup conversion with some help from export-hook. The reason export-hook would probably need to be involved is that such a conversion currently needs to live in the Semigroup companion object, which lives in Algebra while SemigroupK lives in Cats. I suppose another option is to move SemigroupK into Algebra. I'm curious what @non thinks about these possibilities. But I think we should move forward with this PR for now and talk about the other possibilities as long-term possibilities.

[non]

I think supporting export-hook in algebra is a good idea, but I think we should move forward in Cats until that is all set up. 👍

[non]

@liff I think the checkAll calls you made are fine. They will test that the semigroups work.