Break `abs` into two aspects

[Henning Thielemann]

On Tue, 28 Jan 2020, Dannyu NDos wrote:

> 3. Step 2:
> 
> Second, I suggest to move `abs` and `signum` from `Num` to `Floating`:
> 
>     class Floating a where
>          abs :: a -> a
>          signum :: a -> a
>          ...

Rational is not Floating, but has reasonable abs and signum.