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)
```

Start by reviewing the proposed Haskell `Mod` definition and the requested `Semigroup` instance in the issue. Confirm the intended modular-arithmetic semantics and project location before proceeding; the issue is complete only when the maintainers agree on the design and the instance is integrated with appropriate verification.

Written by the indexing model from the issue text.