Fully support strictness flags in data declarations

[mabre]

The following program works (ie. results in java.lang.ArithmeticException: / by zero) with Frege:

data S = S {
    !a :: Int,
     b :: Int
}

main :: IO Int
main = return $ r f
    where
        f = S (1 `div` 0) 2
        r (S _ b) = b

(Note that differing from the Haskell standard !a :: Int is used and not a :: !Int).
But Frege does not support strictness flags without fields, ie. data S = S !Int Int does not work.

[Ingo60]

Strictly speaking, bang patterns are not standard. However, it'd be good if frege would accept that syntax, too.