Add derived Show instances to Language.C.Data

[ezyang]

It would be quite useful to have derived Show instances for Language.C.Data, so that there's an easy way to take a look at the AST that Language.C parses into.

[benedikt]

Patch: Show instances for AST types

[benedikt]

(from Alexander Bernauer)

Please find attached a patch whitch enables dumping the AST of a parsed
file.

Basically all data types from the Syntax module are now deriving Show.
Additionally, Data.NodeInfo? has a manually created instance of Show which
just outputs "_". If somebody want's to really show a NodeInfo? he/she
has to wrap it into the NodeInfoS type which has a proper Show
instanciation.

I hope doing it this way is acceptable. And I hope I didn't mess
anything up with darcs. This is my first project where I am using it.