Add 'Semigroup' and 'Monoid' instances for 'Segment'

These instance would be really useful!

```haskell
instance Semigroup r => Semigroup (Segment r) where
    (<>) = liftA2 (<>)

instance Monoid r => Monoid (Segment r) where
    mempty  = pure mempty
    mappend = liftA2 mappend
```