Proposal: remove datatype contexts from base

[Edward Z. Yang]

+1

Excerpts from Ross Paterson's message of Tue May 24 10:08:14 -0400 2011:
> The Haskell' committee decided to remove datatype contexts from the language:
> 
> http://www.haskell.org/pipermail/haskell-prime/2011-January/003335.html
> 
> (though this doesn't seem to be in the Report yet.)
> 
> In the base package there are 4 of these:
> 
>   Data/Complex.hs:data (RealFloat a) => Complex a
>   GHC/Arr.lhs:data Ix i => Array i e
>   GHC/Real.lhs:data  (Integral a)      => Ratio a = !a :% !a  deriving (Eq)
>   Control/Arrow.hs:newtype ArrowApply a => ArrowMonad a b = ArrowMonad (a () b)
> 
> As far as I know, removing them won't break any code that compiles now,
> so let's do it.
>