Add Arch and OS instances of Pretty

[mpilgrem]

The motivation is that the Stack project can make use of the instances and this will avoid orphan instances.

rio-prettyprint already has a dependency on the Cabal package and, consequently, the Cabal-syntax package.

The instances mirror those of Cabal-syntax for its Distribution.Pretty.Pretty class. That class is based on the pretty package's Text.PrettyPrint.Doc type (a newtype wrapper around its Text.PrettyPrint.Annotated.HughesPJ.Doc type).

The rio-prettyprint package's StyleDoc type is based on the annotated-wl-pprint package's Text.PrettyPrint.Annotated.Leijen.Doc a type.