Modulo Semigroup?

Would you guys be interested in a PR adding a `Semigroup` for modular arithmetic?

Something like this:
``` haskell
data Mod (k :: Nat) (f :: * -> *) (a :: *) = Mod (f a)

instance (KnownNat k, Semigroup (f a), Applicative f,  Integral a) => Semigroup (Mod k f a) where
  (<>) (Mod fx) (Mod fy) = Mod $
    mod <$> (fx <> fy) <*> (pure $ fromIntegral $ natVal $ Proxy @k)
```