Frown: c2hs parser

Copperbox revision 55.

The pretty printer has been substantially revised, it should now better handle 'empty' documents.

To do this I've introduced a new pretty printing library based on Daan Leijen's PPrint but which is 'empty biased':

Paraphrasing Haskell into something I can type into Blogger:

empty `space` doc results in (doc), whereas with the usual pretty printers it would result in (space, doc).

At the moment the output is less pretty than the previous revision and the new infix combinators have symbols that may be changed, but overall I think it is a good bit simpler for printing C where there are many AST elements that have optional properties.