Add "source" button for instances in Hackage

[polkovnikov.ph]

There's no "Source" button to the right of the TC instance documentation in Hackage.

I can see there're some structural problems with source navigation in Hackage (i.e. wrong files shown when dealing with preprocessor). I suppose it happens, because the only file it looks into is the one currently opened. This may be a real problem due to the scoping of instances. I'm rather unsure whether I should file an another ticket for this.

[igloo]

Thanks for the report.

However, haddock issues should be filed in  the hadock trac. Or if the problem is with Hackage rather than haddock, please use the  Cabal github bugtracker.