Support KindSignatures

[ndmitchell]

As raised in an HLint bug: https://code.google.com/p/ndmitchell/issues/detail?id=630

{-# LANGUAGE GADTs, DataKinds, KindSignatures #-}
module HLTest where
data Nat = Z | S Nat
data Shrunk (n :: Nat) a where
  H :: int -> Shrunk (S n) a

Parsing fails.

[hesselink]

I don't think this is because KindsSignatures aren't supported (they seem to parse fine here). The unsupported part is DataKinds, see #19.

[ndmitchell]

In that case #19 and #67 subsume this error, so I'll close this one.