re-export Data.Monoid.<> when available

[aavogt]

I don't think many programs break if <> changes from Doc -> Doc -> Doc to Monoid m => m -> m -> m, since everything else in the library has type Doc. The fixity is the same between the two versions.

This change will save people from having to write hiding ((<>))

[hvr]

please migrate to https://github.com/ekmett/ansi-wl-pprint/issues