[PATCH][RFC] Avoid table lookup in isascii, isxdigit.

[Paul Eggert]

On 04/12/2013 11:43 PM, Rich Felker wrote:
> Then make it ((unsigned)(int)(c)-'0'<10).

On further thought, that's dubious too.
An implementation is required to diagnose a call
like 'isdigit ("")' but the above implementation
wouldn't necessarily do that.

I think Ondřej's original implementation should work fine.
(As a style issue, I prefer to omit the unnecessary parentheses,
though, and I don't understand why the change added them.)