Add instance IsString ShowS where fromString = showString

[Ross Paterson]

On Sun, Sep 27, 2009 at 10:51:04PM +0200, Bas van Dijk wrote:
> It would be really nice to use OverloadedStrings when constructing a
> ShowS, for example in a definition of showsPrec in an instance for
> Show.
> 
> The patch attached in the ticket[3544] adds the necessary instance to
> base/GHC/Show.lhs:
> 
> instance IsString ShowS where
>     fromString = showString
> 
> Note that this does require TypeSynonymInstances.

and also FlexibleInstances.  I think that's a problem, as Ganesh outlined.