Exists2

In `Sort` all non-primitive type applications 

     (t1 t2 ... tn)

are encoded as 

     FApp( "FAppTy", [FApp ("FAppTy", [t1, t2]),...), tn])

See commentary at: 

   https://github.com/ucsd-progsys/liquidhaskell/pull/149

Also linked to `liquid-fixpoint` pull request ZZZ