- The (non-exported) prelude is in prelude/Prelude.hs.
- It exports Monoid and Applicative, like base 4.8 prelude,
but works with older base versions.
- It exports (<>) for mappend.
- It hides 'catch' on older base versions.
This allows us to remove many imports of Data.Monoid
and Control.Applicative, and remove Text.Pandoc.Compat.Monoid.
It should allow us to use -Wall again for ghc 7.10.
