-Wall -Werror

[Ashley Yakeley]

Simon Peyton-Jones wrote:
> That's fine.  But then you can choose
> 
> a) add -fno-warn-orphans
> or
> b) move the (Show Day) instance to the module declaring Day.
> 
> I'm not sure which is best for you, but you're the package author so you get to decide!

Patch pushed. I've plumped for option a. It's better structuring, as b 
would involve moving more code from Gregorian.hs to Days.hs.

The concern Henning raised shouldn't apply, as both modules are hidden 
and re-exported by Data.Time.Calendar. I've also fixed two other modules 
with the same issue (again, also hidden).

-- 
Ashley Yakeley