`Semigroup` and `Monoid`: clean-up and improvements 

- Add more instances of `Semigroup` and `Monoid` to mimic instances defined in `Data.Monoid` and `Data.Semigroup` in `base`
- Do not re-export the whole `Data.Semigroup` module in `Data.Monoid.Linear`, but rather, export only `newtype`s, and add linear `Semigroup` and `Monoid` instances to those types