default instance for IsString

[Yitzchak Gale]

Markus Läll wrote:
> You do know, that you already *can* have safe Text and ByteString from
> an overloaded string literal.

Yes, the IsString instances for Text and ByteString are safe
(I hope).

But in order to use them, I have to turn on OverloadedStrings.
That could cause other string literals in the same module
to throw exceptions at run time.

-Yitz