[R6RS] fixnum operators and overflow detection

[dyb at cs.indiana.edu]

> If we accept this precedent, then fx+ should signal on
> overflow.

That would be great.

> Shall we use fixnum+ as name for the version
> that wraps modulo 2^N?

fixnum+ is fine for +, -, and * but not so great for the shift operators. 
Should we use a fixnum- prefix for those?  Or perhaps we can slip some C
into the mix and use a fx% prefix for all of the wrapping operators.

By the way, both kinds of left shift should have been included in my list. 
I was confused about the shift operators when I wrote the first note.

Kent