[R6RS] fixnum operators and overflow detection

[dyb at cs.indiana.edu]

We've talked about this before but without any resolution.

I would prefer that the fixnum operators, including at least fx+, fx*,
fx-, and the fx shift left operator(s), signal an error when the exact
representation cannot be represented as a fixnum.  This is the way they
work in Chez Scheme, and this has caught several errors that might
otherwise have gone undetected otherwise.

Kent