Remove GHC.Err import from Data.Maybe

[Simon Marlow]

Judah Jacobson wrote:
> On Dec 31, 2007 11:57 AM, Neil Mitchell <ndmitchell at gmail.com> wrote:
>> These imports of Err all include {-# SOURCE #-} pragmas, which GHC
>> warns me are unnecessary, but clear change the output. What do the
>> SOURCE pragma's do? I couldn't find them in the manual.
>>
> 
> They break the circular dependency between recursive modules; every
> module imported with {-# SOURCE #-} must have a related .hs-boot file.
>  The manual describes this in section 5.6.9:
> http://www.haskell.org/ghc/docs/latest/html/users_guide/separate-compilation.html#mutual-recursion

SOURCE pragmas should be mentioned in the section on pragmas in the docs; 
I'll fix that.

Cheers,
	Simon