Add instance Storable Ordering

[Sylvain Henry]

A `Word8` would be enough instead of 4 or 8 bytes for `CInt`.

On 08/10/2019 17:33, Sven Panne wrote:
> Am Di., 8. Okt. 2019 um 16:57 Uhr schrieb Simon Jakobi via Libraries 
> <libraries at haskell.org <mailto:libraries at haskell.org>>:
>
>     That seems like a good idea! [...]
>
>
> Looks OK, but let's use toEnum in peekElemOff:
>
>   instance Storable Ordering where
>     sizeOf _ = sizeOf (undefined :: CInt)
>     alignment _ = alignment (undefined :: CInt)
>     peekElemOff p i = toEnum . fromIntegral <$> peekElemOff (castPtr p 
> :: Ptr CInt) i
>     pokeElemOff p i = pokeElemOff (castPtr p :: Ptr CInt) i . 
> fromIntegral . fromEnum
>
> This is more symmetrical with pokeElemOff and less obfuscated. Note 
> that the instance works for every Enum, so if there are more cases 
> like Ordering, it might be worth to abstract that out.
>
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