In all this discussion, it seems that we have not discussed the
possibility of adapting David Gay's code, dtoa.c, which nicely handles
both halves of the problem.  It's also free and has been well exercised
over the years.

It's available here: http://www.netlib.org/fp/

It'd probably have to be touched up a bit.  Guido notes that, for
example, malloc is called without checking the return value. 
Nevertheless, the core is what we're looking for, I believe.

It's described in the paper: "Correctly rounded binary-decimal and
decimal-binary conversions", David M. Gay

It comes recommended by Steele and White (output) and by Clinger
(input), in the retrospectives on their seminal papers.