ivory lang: need overlapping instances in Ivory.Language.Init
ivory lang: need overlapping instances in Ivory.Language.Init

* previously, ghc won't be able to match IvoryZero to Stored IBool,
  complaining the explicit Stored IBool instance overlaps with the
  (Num a, IvoryInit a) => Stored a instance, because the Num class
  not necessarily closed.

GaloisInc/ivory,Hodapp87/ivory,GaloisInc/ivory