Don't use custom prelude for latest ghc.
This is a better approach to making 'stack ghci' and 'cabal repl'
work. Instead of using NoImplicitPrelude, we only use the custom
prelude for older ghc versions. The custom prelude presents a
uniform API that matches the current base version's prelude.
So, when developing (presumably with latest ghc), we don't
use a custom prelude at all and hence have no trouble with ghci.
The custom prelude no longer exports (<>): we now want to
match the base 4.8 prelude behavior.
- 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.
