Implement the signbit() function

[Michael D. Godfrey]

I looked some more at mappers.cc, but could not
understand it well enough to be confident about
implementing a signbit() function whose behavior would
be exactly like sign() but it would return the signbit(s)
of its argument, and it can directly use the signbit in C++.
Implementing this function should be
fairly easy for someone with a good understanding
of mappers.cc.

The most obvious place where signbit() would be useful,
for me, is when using atan2.  The sign is needed to decide
on the correct quadrant where atan2 is +- 0..  Doing this now
is definitely cumbersome.  It is easy to think of other applications.

So, if you agree that this is useful, can we find someone to
do it?

Michael