[R6RS] fixnum operators and overflow detection

[William D Clinger]

Mike wrote:
> OK by me.  Are you making that change or should I?

I've already made the change.  Since you have agreed
to the changes I want to make, and no one else has
yet objected, I'll go ahead and make those changes
to the arithmetic SRFI.  I should finish the SRFI by
tomorrow at the latest, and will work on renaming in
the reference implementation over the weekend.

> > Should we have fixnum<, or fx<, or both?
>
> Both.

> > Most importantly, I'd like to know the rationale for
> > restricting the domains of fx+/carry, fx-/carry, and
> > fx*/carry to non-negative fixnums, and for requiring
> > them to return non-negative fixnums as results.
>
> ....I have no objection to generalizing it.

Will