rpn 5.17.3

Small improvement to unit conversion.  It only reduces units when it has
to.  Otherwise, converting cups/minute to teaspoons/second ends up going
through cubic meters unnecessarily.   Plus, it didn't give the right
answer.

It's slowly getting better, and handles all the basic conversion I throw
at it.  The next step is to add more intelligence concerning
equivalences between different types of units (e.g., applying Ohm's
law).  That will be more of a challenge.