bug#30408: Checking for loss of information on integer conversion

[Eli Zaretskii <eliz@gnu.org>]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Tue, 27 Mar 2018 16:19:21 -0700
> 
> Here's a patch that I hope addresses the main problem. The basic idea is 
> to avoid the confusion exemplified in Bug#30408 by changing Emacs so 
> that it ordinarily signals an error if it reads a program that contains 
> an integer literal that is out of fixnum range. However, if the 
> out-of-range literal is followed by '.' then Emacs continues to silently 
> convert it to floating-point; this is intended as an escape hatch for 
> any programs that need the old behavior (I expect this'll be rare). 

I'd suggest, for a good measure, to have a variable which would force
the conversion to floats, avoiding an error even without the trailing
period.  We can later remove that variable, or make it a no-op, if the
danger of breaking existing code turns out low or non-existent.