HsSyn clean up for indexed types
- This patch cleans up the HsSyn representation of type family declarations. - The new representation is not only less delicate, it also simplified teh code a bit. - I took the opportunity of stream lining the terminology and function names at the same time. - I also updated the description on the wiki at <http://hackage.haskell.org/trac/ghc/wiki/TypeFunctionsSyntax>