make Hashable instance for PVar same as Eq instance
make Hashable instance for PVar same as Eq instance

ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ssaavedra/liquidhaskell,ssaavedra/liquidhaskell,ucsd-progsys/liquidhaskell,ssaavedra/liquidhaskell,ucsd-progsys/liquidhaskell,ssaavedra/liquidhaskell,ucsd-progsys/liquidhaskell,ssaavedra/liquidhaskell,ssaavedra/liquidhaskell,ucsd-progsys/liquidhaskell