Deriving instance for Foreign.C.Error.Errno (trac 2261)

[Thomas M. DuBuisson]

Errno, the Haskell encapsulation of the CInt 'errno' value, should be an
instance of the commonly used type classes so higher level data
structures that include it aren't bothered.

This change will break code that has custom instances for Errno, but
this should be little (if any) code.

A patch is attached to the trac ticket to derive Show and Ord while Eq
remains a custom instance.  One could argue that, like Eq, Show and Ord
should be customized to provide a identical result for all 'invalid'
errno's - any thoughts on if this is necessary and what the corner case
result would be?

http://hackage.haskell.org/trac/ghc/ticket/2261

Thomas

P.S. Traveling soon, so I won't be responsive - sorry.