Some refactoring in formula types

[jvoigtlaender]

Wouldn't this:

logic-tasks/src/Formula/Types.hs

Lines 84 to 86 in 99ebf3f

	data Literal
	= Literal { letter :: Char} -- ^ positive sign
	| Not { letter :: Char} -- ^ negative sign

better be given constructors as follows?

data Literal
    = Pos { letter :: Char} -- ^ positive sign
    | Neg { letter :: Char} -- ^ negative sign

Also, instead of

logic-tasks/src/Formula/Types.hs

Lines 59 to 61 in 99ebf3f

	class Formula a where
	literals :: a -> [Literal]
	atomics :: a -> [Literal]

have the following?

class Formula a where
    literals :: a -> [Literal]
    atomics :: a -> [Char]