Add instance Storable Ordering

[Sven Panne]

Am Di., 8. Okt. 2019 um 16:57 Uhr schrieb Simon Jakobi via Libraries <
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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