Display minimal complete definitions for type classes

This corresponds to the new {-# MINIMAL #-} pragma present in GHC 7.8+.
I also cleaned up some of the places in which ExportDecl is used to make
adding fields easier in the future.

NOTE: The method for extracting minimal definition information from
modules is currently very less-than-satisfactory, see the comment at
src/Haddock/Interface/Create.hs:95.

Lots of test cases have been updated since they now render with
minimality information.