Improve Data instance

[rl]

The current Data instance for vectors doesn't define toConstr and gunfold. This breaks some libraries, most notable uniplate. We should define the Data instance like this:

instance ... => Data (Vector a) where
  gunfold k z c
    | constrIndex c == 1 = k (z fromList)
    | otherwise = error ...

  toConstr v = v `seq` vConstr

  dataTypeOf _ = vType

vConstr = mkConstr vType "fromList" [] Prefix
vType = mkDataType "Vector" [vConstr]